diff --git a/lib/mongo/cluster.rb b/lib/mongo/cluster.rb
index 933d80a6b5..2134d5ec8c 100644
--- a/lib/mongo/cluster.rb
+++ b/lib/mongo/cluster.rb
@@ -37,6 +37,11 @@ class Cluster
     # @since 2.1.1
     READ_RETRY_INTERVAL = 5
 
+    # How often an idle primary writes a no-op to the oplog.
+    #
+    # @since 2.4.0
+    IDLE_WRITE_PERIOD_SECONDS = 10
+
     # @return [ Hash ] The options hash.
     attr_reader :options
 
diff --git a/lib/mongo/error/invalid_server_preference.rb b/lib/mongo/error/invalid_server_preference.rb
index 441b6d16da..eb7c87549a 100644
--- a/lib/mongo/error/invalid_server_preference.rb
+++ b/lib/mongo/error/invalid_server_preference.rb
@@ -33,7 +33,9 @@ class InvalidServerPreference < Error
       # Error message for when the max staleness is not at least twice the heartbeat frequency.
       #
       # @since 2.4.0
-      INVALID_MAX_STALENESS = "max_staleness must be at least twice the client's heartbeat frequency.".freeze
+      INVALID_MAX_STALENESS = "`max_staleness` value is too small. It must be at least " +
+        "`ServerSelector::SMALLEST_MAX_STALENESS_SECONDS` and (the cluster's heartbeat_frequency " +
+          "setting + `Cluster::IDLE_WRITE_PERIOD_SECONDS`).".freeze
 
       # Error message when max staleness cannot be used because one or more servers has version < 3.4.
       #
diff --git a/lib/mongo/server.rb b/lib/mongo/server.rb
index a8ede780a8..4bfe1f341e 100644
--- a/lib/mongo/server.rb
+++ b/lib/mongo/server.rb
@@ -45,6 +45,7 @@ class Server
 
     # Get the description from the monitor and scan on monitor.
     def_delegators :monitor, :description, :scan!, :heartbeat_frequency, :last_scan
+    alias :heartbeat_frequency_seconds :heartbeat_frequency
 
     # Delegate convenience methods to the monitor description.
     def_delegators :description,
diff --git a/lib/mongo/server_selector.rb b/lib/mongo/server_selector.rb
index 1d13ad520d..54eeef09da 100644
--- a/lib/mongo/server_selector.rb
+++ b/lib/mongo/server_selector.rb
@@ -38,6 +38,11 @@ module ServerSelector
     # @since 2.0.0
     SERVER_SELECTION_TIMEOUT = 30.freeze
 
+    # The smallest allowed max staleness value, in seconds.
+    #
+    # @since 2.4.0
+    SMALLEST_MAX_STALENESS_SECONDS = 90
+
     # Primary read preference.
     #
     # @since 2.1.0
diff --git a/lib/mongo/server_selector/nearest.rb b/lib/mongo/server_selector/nearest.rb
index 6011617255..b170178c98 100644
--- a/lib/mongo/server_selector/nearest.rb
+++ b/lib/mongo/server_selector/nearest.rb
@@ -67,7 +67,7 @@ def tags_allowed?
       def to_mongos
         preference = { :mode => 'nearest' }
         preference.merge!({ :tags => tag_sets }) unless tag_sets.empty?
-        preference.merge!({ maxStalenessMS: max_staleness * 1000 }) if max_staleness
+        preference.merge!({ maxStalenessSeconds: max_staleness }) if max_staleness
         preference
       end
 
diff --git a/lib/mongo/server_selector/primary_preferred.rb b/lib/mongo/server_selector/primary_preferred.rb
index f82afcb9ab..b8e70a4bf0 100644
--- a/lib/mongo/server_selector/primary_preferred.rb
+++ b/lib/mongo/server_selector/primary_preferred.rb
@@ -68,7 +68,7 @@ def tags_allowed?
       def to_mongos
         preference = { :mode => 'primaryPreferred' }
         preference.merge!({ :tags => tag_sets }) unless tag_sets.empty?
-        preference.merge!({ maxStalenessMS: max_staleness * 1000 }) if max_staleness
+        preference.merge!({ maxStalenessSeconds: max_staleness }) if max_staleness
         preference
       end
 
diff --git a/lib/mongo/server_selector/secondary.rb b/lib/mongo/server_selector/secondary.rb
index 01fb156031..7defaff877 100644
--- a/lib/mongo/server_selector/secondary.rb
+++ b/lib/mongo/server_selector/secondary.rb
@@ -68,7 +68,7 @@ def tags_allowed?
       def to_mongos
         preference = { :mode => 'secondary' }
         preference.merge!({ :tags => tag_sets }) unless tag_sets.empty?
-        preference.merge!({ maxStalenessMS: max_staleness * 1000 }) if max_staleness
+        preference.merge!({ maxStalenessSeconds: max_staleness }) if max_staleness
         preference
       end
 
diff --git a/lib/mongo/server_selector/secondary_preferred.rb b/lib/mongo/server_selector/secondary_preferred.rb
index 5ab4218dbc..e1609135df 100644
--- a/lib/mongo/server_selector/secondary_preferred.rb
+++ b/lib/mongo/server_selector/secondary_preferred.rb
@@ -71,7 +71,7 @@ def to_mongos
         return nil if tag_sets.empty? && max_staleness.nil?
         preference = { mode: 'secondaryPreferred' }
         preference.merge!({ tags: tag_sets }) unless tag_sets.empty?
-        preference.merge!({ maxStalenessMS: max_staleness * 1000 }) if max_staleness
+        preference.merge!({ maxStalenessSeconds: max_staleness }) if max_staleness
         preference
       end
 
diff --git a/lib/mongo/server_selector/selectable.rb b/lib/mongo/server_selector/selectable.rb
index eb976af32b..7df7276368 100644
--- a/lib/mongo/server_selector/selectable.rb
+++ b/lib/mongo/server_selector/selectable.rb
@@ -26,7 +26,7 @@ module Selectable
       # @return [ Array ] tag_sets The tag sets used to select servers.
       attr_reader :tag_sets
 
-      # @return [ Float ] max_staleness The maximum replication lag, in seconds, that a
+      # @return [ Integer ] max_staleness The maximum replication lag, in seconds, that a
       #   secondary can suffer and still be eligible for a read.
       #
       # @since 2.4.0
@@ -68,7 +68,7 @@ def ==(other)
       def initialize(options = {})
         @options = (options || {}).freeze
         @tag_sets = (options[:tag_sets] || []).freeze
-        @max_staleness = options[:max_staleness] if options[:max_staleness] && options[:max_staleness] > 0
+        @max_staleness = options[:max_staleness] unless options[:max_staleness] == -1
         validate!
       end
 
@@ -154,7 +154,7 @@ def candidates(cluster)
         elsif cluster.sharded?
           near_servers(cluster.servers).each { |server| validate_max_staleness_support!(server) }
         else
-          validate_max_staleness_value!(cluster)
+          validate_max_staleness_value!(cluster) unless cluster.unknown?
           select(cluster.servers)
         end
       end
@@ -230,14 +230,14 @@ def filter_stale_servers(candidates, primary = nil)
             validate_max_staleness_support!(server)
             staleness = (server.last_scan - server.last_write_date) -
                         (primary.last_scan - primary.last_write_date)  +
-                        (server.heartbeat_frequency * 1000)
+                        (server.heartbeat_frequency_seconds * 1000)
             staleness <= max_staleness_ms
           end
         else
           max_write_date = candidates.collect(&:last_write_date).max
           candidates.select do |server|
             validate_max_staleness_support!(server)
-            staleness = max_write_date - server.last_write_date + (server.heartbeat_frequency * 1000)
+            staleness = max_write_date - server.last_write_date + (server.heartbeat_frequency_seconds * 1000)
             staleness <= max_staleness_ms
           end
         end
@@ -258,10 +258,12 @@ def validate_max_staleness_support!(server)
       end
 
       def validate_max_staleness_value!(cluster)
-        return unless @max_staleness
-        heartbeat_frequency = cluster.options[:heartbeat_frequency] || Server::Monitor::HEARTBEAT_FREQUENCY
-        if @max_staleness < heartbeat_frequency * 2
-          raise Error::InvalidServerPreference.new(Error::InvalidServerPreference::INVALID_MAX_STALENESS)
+        if @max_staleness
+          heartbeat_frequency_seconds = cluster.options[:heartbeat_frequency] || Server::Monitor::HEARTBEAT_FREQUENCY
+          unless @max_staleness >= [ SMALLEST_MAX_STALENESS_SECONDS,
+                                     (heartbeat_frequency_seconds  + Cluster::IDLE_WRITE_PERIOD_SECONDS) ].max
+            raise Error::InvalidServerPreference.new(Error::InvalidServerPreference::INVALID_MAX_STALENESS)
+          end
         end
       end
     end
diff --git a/lib/mongo/uri.rb b/lib/mongo/uri.rb
index 16ba96d814..699e534368 100644
--- a/lib/mongo/uri.rb
+++ b/lib/mongo/uri.rb
@@ -374,7 +374,7 @@ def self.uri_option(uri_key, name, extra = {})
     # Read Options
     uri_option 'readpreference', :mode, :group => :read, :type => :read_mode
     uri_option 'readpreferencetags', :tag_sets, :group => :read, :type => :read_tags
-    uri_option 'maxstalenessms', :max_staleness, :group => :read, :type => :ms_convert
+    uri_option 'maxstalenessseconds', :max_staleness, :group => :read
 
     # Pool options
     uri_option 'minpoolsize', :min_pool_size
diff --git a/spec/mongo/max_staleness_spec.rb b/spec/mongo/max_staleness_spec.rb
index c028b3d5e2..78fd012faa 100644
--- a/spec/mongo/max_staleness_spec.rb
+++ b/spec/mongo/max_staleness_spec.rb
@@ -26,10 +26,10 @@
         if spec.heartbeat_frequency
           TEST_OPTIONS.merge(heartbeat_frequency: spec.heartbeat_frequency)
         else
-          copy = TEST_OPTIONS.dup
-          copy.delete(:heartbeat_frequency)
-          copy
-        end.merge!(server_selection_timeout: 0.2)
+          TEST_OPTIONS.dup.tap do |opts|
+            opts.delete(:heartbeat_frequency)
+          end
+        end.merge!(server_selection_timeout: 0.2, connect_timeout: 0.1)
       end
 
       let(:cluster) do
@@ -38,7 +38,8 @@
           allow(c).to receive(:single?).and_return(topology.single?)
           allow(c).to receive(:sharded?).and_return(topology.sharded?)
           allow(c).to receive(:replica_set?).and_return(topology.replica_set?)
-          allow(c).to receive(:options).and_return(options.merge(server_selection_timeout: 0.2))
+          allow(c).to receive(:unknown?).and_return(topology.unknown?)
+          allow(c).to receive(:options).and_return(options)
           allow(c).to receive(:scan!).and_return(true)
           allow(c).to receive(:app_metadata).and_return(app_metadata)
         end
@@ -47,17 +48,17 @@
       let(:candidate_servers) do
         spec.candidate_servers.collect do |server|
           features = double('features').tap do |feat|
-            allow(feat).to receive(:max_staleness_enabled?).and_return(server['maxWireVersion'] >= 5)
+            allow(feat).to receive(:max_staleness_enabled?).and_return(server['maxWireVersion'] && server['maxWireVersion'] >= 5)
           end
           address = Mongo::Address.new(server['address'])
           Mongo::Server.new(address, cluster, monitoring, listeners, options).tap do |s|
-            allow(s).to receive(:average_round_trip_time).and_return(server['avg_rtt_ms'] / 1000.0)
+            allow(s).to receive(:average_round_trip_time).and_return(server['avg_rtt_ms'] / 1000.0) if server['avg_rtt_ms']
             allow(s).to receive(:tags).and_return(server['tags'])
             allow(s).to receive(:secondary?).and_return(server['type'] == 'RSSecondary')
             allow(s).to receive(:primary?).and_return(server['type'] == 'RSPrimary')
             allow(s).to receive(:connectable?).and_return(true)
-            allow(s).to receive(:last_write_date).and_return(server['lastWrite']['lastWriteDate']['$numberLong'].to_i * 1000)
-            allow(s).to receive(:last_scan).and_return(server['lastUpdateTime'] * 1000)
+            allow(s).to receive(:last_write_date).and_return(server['lastWrite']['lastWriteDate']['$numberLong'].to_i) if server['lastWrite']
+            allow(s).to receive(:last_scan).and_return(server['lastUpdateTime'])
             allow(s).to receive(:features).and_return(features)
           end
         end
@@ -65,7 +66,7 @@
 
       let(:in_latency_window) do
         spec.in_latency_window.collect do |server|
-          Mongo::Server.new(Mongo::Address.new(server['address']), cluster, monitoring, listeners)
+          Mongo::Server.new(Mongo::Address.new(server['address']), cluster, monitoring, listeners, options)
         end
       end
 
@@ -84,23 +85,36 @@
         allow(cluster).to receive(:servers).and_return(candidate_servers)
       end
 
-      context 'Valid read preference and matching server available', unless: spec.invalid_max_staleness? do
+      context 'when the max staleness is invalid' do
 
-        it 'Finds all suitable servers in the latency window', if: spec.replica_set? do
-          expect(server_selector.send(:select, cluster.servers)).to match_array(in_latency_window)
-        end
+        it 'Raises an InvalidServerPreference exception', if: spec.invalid_max_staleness? do
 
-        it 'Finds the most suitable server in the latency window' do
-          expect(in_latency_window).to include(server_selector.select_server(cluster))
+          expect do
+            server_selector.select_server(cluster)
+          end.to raise_exception(Mongo::Error::InvalidServerPreference)
         end
       end
 
-      context 'when the max staleness cannot be applied', if: spec.invalid_max_staleness? do
+      context 'when the max staleness is valid' do
 
-        it 'Raises exception' do
-          expect do
-            server_selector.select_server(cluster)
-          end.to raise_exception(Mongo::Error::InvalidServerPreference)
+        context 'when there are available servers' do
+
+          it 'Finds all suitable servers in the latency window', if: (spec.replica_set? && !spec.invalid_max_staleness? && spec.server_available?) do
+            expect(server_selector.send(:select, cluster.servers)).to match_array(in_latency_window)
+          end
+
+          it 'Finds the most suitable server in the latency window', if: (!spec.invalid_max_staleness? && spec.server_available?) do
+            expect(in_latency_window).to include(server_selector.select_server(cluster))
+          end
+        end
+
+        context 'when there are no available servers', if: (!spec.invalid_max_staleness? && !spec.server_available?) do
+
+          it 'Raises a NoServerAvailable Exception' do
+            expect do
+              server_selector.select_server(cluster)
+            end.to raise_exception(Mongo::Error::NoServerAvailable)
+          end
         end
       end
     end
diff --git a/spec/mongo/server_selection_spec.rb b/spec/mongo/server_selection_spec.rb
index ab64da3bcd..50b4e55a28 100644
--- a/spec/mongo/server_selection_spec.rb
+++ b/spec/mongo/server_selection_spec.rb
@@ -28,6 +28,7 @@
           allow(c).to receive(:single?).and_return(topology.single?)
           allow(c).to receive(:sharded?).and_return(topology.sharded?)
           allow(c).to receive(:replica_set?).and_return(topology.replica_set?)
+          allow(c).to receive(:unknown?).and_return(topology.unknown?)
           allow(c).to receive(:app_metadata).and_return(app_metadata)
         end
       end
diff --git a/spec/mongo/server_selector/nearest_spec.rb b/spec/mongo/server_selector/nearest_spec.rb
index 1336b5f158..e6fe31d17d 100644
--- a/spec/mongo/server_selector/nearest_spec.rb
+++ b/spec/mongo/server_selector/nearest_spec.rb
@@ -18,7 +18,7 @@
     context 'when max_staleness is provided' do
 
       let(:options) do
-        { max_staleness: 60 }
+        { max_staleness: 95 }
       end
 
       it 'sets the max_staleness option' do
@@ -32,7 +32,7 @@
     context 'when max staleness is the same' do
 
       let(:options) do
-        { max_staleness: 60 }
+        { max_staleness: 95 }
       end
 
       let(:other) do
@@ -47,7 +47,7 @@
     context 'when max staleness is different' do
 
       let(:other_options) do
-        { max_staleness: 30 }
+        { max_staleness: 100 }
       end
 
       let(:other) do
@@ -100,11 +100,11 @@
     context 'max staleness provided' do
 
       let(:max_staleness) do
-        60
+        100
       end
 
       let(:expected) do
-        { :mode => 'nearest', maxStalenessMS: 60000 }
+        { :mode => 'nearest', maxStalenessSeconds: 100 }
       end
 
       it 'returns a read preference formatted for mongos' do
diff --git a/spec/mongo/server_selector/primary_preferred_spec.rb b/spec/mongo/server_selector/primary_preferred_spec.rb
index a7ed464d30..52070df3de 100644
--- a/spec/mongo/server_selector/primary_preferred_spec.rb
+++ b/spec/mongo/server_selector/primary_preferred_spec.rb
@@ -18,7 +18,7 @@
     context 'when max_staleness is provided' do
 
       let(:options) do
-        { max_staleness: 60 }
+        { max_staleness: 95 }
       end
 
       it 'sets the max_staleness option' do
@@ -32,7 +32,7 @@
     context 'when max staleness is the same' do
 
       let(:options) do
-        { max_staleness: 60 }
+        { max_staleness: 95 }
       end
 
       let(:other) do
@@ -47,7 +47,7 @@
     context 'when max staleness is different' do
 
       let(:other_options) do
-        { max_staleness: 30 }
+        { max_staleness: 100 }
       end
 
       let(:other) do
@@ -93,11 +93,11 @@
     context 'max staleness provided' do
 
       let(:max_staleness) do
-        60
+        100
       end
 
       let(:expected) do
-        { :mode => 'primaryPreferred', maxStalenessMS: 60000 }
+        { :mode => 'primaryPreferred', maxStalenessSeconds: 100 }
       end
 
       it 'returns a read preference formatted for mongos' do
diff --git a/spec/mongo/server_selector/primary_spec.rb b/spec/mongo/server_selector/primary_spec.rb
index 61c62e9cbc..94cc0ae358 100644
--- a/spec/mongo/server_selector/primary_spec.rb
+++ b/spec/mongo/server_selector/primary_spec.rb
@@ -16,7 +16,7 @@
     context 'when max_staleness is provided' do
 
       let(:options) do
-        { max_staleness: 60 }
+        { max_staleness: 100 }
       end
 
       it 'raises an exception' do
@@ -66,7 +66,7 @@
     context 'max staleness provided' do
 
       let(:max_staleness) do
-        60
+        100
       end
 
       it 'raises an error' do
diff --git a/spec/mongo/server_selector/secondary_preferred_spec.rb b/spec/mongo/server_selector/secondary_preferred_spec.rb
index 6fd1205d76..9faeaa95d3 100644
--- a/spec/mongo/server_selector/secondary_preferred_spec.rb
+++ b/spec/mongo/server_selector/secondary_preferred_spec.rb
@@ -18,7 +18,7 @@
     context 'when max_staleness is provided' do
 
       let(:options) do
-        { max_staleness: 60 }
+        { max_staleness: 95 }
       end
 
       it 'sets the max_staleness option' do
@@ -32,7 +32,7 @@
     context 'when max staleness is the same' do
 
       let(:options) do
-        { max_staleness: 60 }
+        { max_staleness: 90 }
       end
 
       let(:other) do
@@ -47,7 +47,7 @@
     context 'when max staleness is different' do
 
       let(:other_options) do
-        { max_staleness: 30 }
+        { max_staleness: 100 }
       end
 
       let(:other) do
@@ -100,7 +100,7 @@
       end
 
       let(:expected) do
-        { :mode => 'secondaryPreferred', maxStalenessMS: 60000 }
+        { :mode => 'secondaryPreferred', maxStalenessSeconds: 60 }
       end
 
       it 'returns a read preference formatted for mongos' do
diff --git a/spec/mongo/server_selector/secondary_spec.rb b/spec/mongo/server_selector/secondary_spec.rb
index de59d283d6..fe457fee98 100644
--- a/spec/mongo/server_selector/secondary_spec.rb
+++ b/spec/mongo/server_selector/secondary_spec.rb
@@ -18,7 +18,7 @@
     context 'when max_staleness is provided' do
 
       let(:options) do
-        { max_staleness: 60 }
+        { max_staleness: 100 }
       end
 
       it 'sets the max_staleness option' do
@@ -32,7 +32,7 @@
     context 'when max staleness is the same' do
 
       let(:options) do
-        { max_staleness: 60 }
+        { max_staleness: 90 }
       end
 
       let(:other) do
@@ -47,7 +47,7 @@
     context 'when max staleness is different' do
 
       let(:other_options) do
-        { max_staleness: 30 }
+        { max_staleness: 95 }
       end
 
       let(:other) do
@@ -96,7 +96,7 @@
       end
 
       let(:expected) do
-        { :mode => 'secondary', maxStalenessMS: 60000 }
+        { :mode => 'secondary', maxStalenessSeconds: 60 }
       end
 
       it 'returns a read preference formatted for mongos' do
diff --git a/spec/mongo/server_selector_spec.rb b/spec/mongo/server_selector_spec.rb
index 4b1ae086ac..711e467515 100644
--- a/spec/mongo/server_selector_spec.rb
+++ b/spec/mongo/server_selector_spec.rb
@@ -153,6 +153,7 @@
           allow(c).to receive(:servers).and_return(servers)
           allow(c).to receive(:single?).and_return(false)
           allow(c).to receive(:sharded?).and_return(false)
+          allow(c).to receive(:unknown?).and_return(false)
           allow(c).to receive(:scan!).and_return(true)
           allow(c).to receive(:options).and_return(server_selection_timeout: 0.1)
         end
@@ -182,6 +183,7 @@
           allow(c).to receive(:servers).and_return(servers)
           allow(c).to receive(:single?).and_return(false)
           allow(c).to receive(:sharded?).and_return(false)
+          allow(c).to receive(:unknown?).and_return(false)
           allow(c).to receive(:scan!).and_return(true)
           allow(c).to receive(:options).and_return(server_selection_timeout: 0)
         end
@@ -223,6 +225,7 @@
           allow(c).to receive(:servers).and_return(servers)
           allow(c).to receive(:single?).and_return(false)
           allow(c).to receive(:sharded?).and_return(false)
+          allow(c).to receive(:unknown?).and_return(false)
           allow(c).to receive(:scan!).and_return(true)
           allow(c).to receive(:options).and_return(local_threshold: 0.050)
         end
@@ -251,6 +254,7 @@
           allow(c).to receive(:servers).and_return(servers)
           allow(c).to receive(:single?).and_return(single)
           allow(c).to receive(:sharded?).and_return(sharded)
+          allow(c).to receive(:unknown?).and_return(false)
           allow(c).to receive(:scan!).and_return(true)
           allow(c).to receive(:options).and_return(server_selection_timeout: 0.1)
         end
diff --git a/spec/mongo/uri_spec.rb b/spec/mongo/uri_spec.rb
index e8bd11367e..4c4ef0b77c 100644
--- a/spec/mongo/uri_spec.rb
+++ b/spec/mongo/uri_spec.rb
@@ -541,7 +541,7 @@
     context 'read preference max staleness option provided' do
 
       let(:options) do
-        'readPreference=Secondary&maxStalenessMS=120000'
+        'readPreference=Secondary&maxStalenessSeconds=120'
       end
 
       let(:read) do
@@ -561,7 +561,7 @@
         context 'when max staleness is combined with read preference mode primary' do
 
           let(:options) do
-            'readPreference=primary&maxStalenessMS=120000'
+            'readPreference=primary&maxStalenessSeconds=120'
           end
 
           it 'raises an exception when read preference is accessed on the client' do
@@ -571,10 +571,10 @@
           end
         end
 
-        context 'when the max staleness value is not at least twice heartbeat frequency' do
+        context 'when the max staleness value is too small' do
 
           let(:options) do
-            'readPreference=secondary&maxStalenessMS=1'
+            'readPreference=secondary&maxStalenessSeconds=89'
           end
 
           it 'does not raise an exception until the read preference is used' do
diff --git a/spec/support/max_staleness/ReplicaSetNoPrimary/Incompatible.yml b/spec/support/max_staleness/ReplicaSetNoPrimary/Incompatible.yml
index e987a6d837..74032747ec 100644
--- a/spec/support/max_staleness/ReplicaSetNoPrimary/Incompatible.yml
+++ b/spec/support/max_staleness/ReplicaSetNoPrimary/Incompatible.yml
@@ -1,6 +1,6 @@
-# During server selection,
-# clients (drivers or mongos) MUST raise an error if ``maxStalenessMS`` is not zero or null,
-# and any server's ``maxWireVersion`` is less than 5 (`SERVER-23893`_).
+# During server selection, clients (drivers or mongos) MUST raise an error if
+# maxStalenessSeconds is defined and not -1 and any server's ``maxWireVersion``
+# is less than 5 (`SERVER-23893`_).
 ---
 topology_description:
   type: ReplicaSetNoPrimary
@@ -21,5 +21,5 @@ topology_description:
     lastWrite: {lastWriteDate: {$numberLong: "1"}}
 read_preference:
   mode: Nearest
-  maxStalenessMS: 120000
+  maxStalenessSeconds: 120
 error: true
diff --git a/spec/support/max_staleness/ReplicaSetNoPrimary/LastUpdateTime.yml b/spec/support/max_staleness/ReplicaSetNoPrimary/LastUpdateTime.yml
index 17b922e67f..3acc594c71 100644
--- a/spec/support/max_staleness/ReplicaSetNoPrimary/LastUpdateTime.yml
+++ b/spec/support/max_staleness/ReplicaSetNoPrimary/LastUpdateTime.yml
@@ -7,14 +7,14 @@ topology_description:
     type: RSSecondary
     avg_rtt_ms: 5
     lastUpdateTime: 1
-    lastWrite: {lastWriteDate: {$numberLong: "25002"}}
+    lastWrite: {lastWriteDate: {$numberLong: "125002"}}
     maxWireVersion: 5
   - &2
     address: b:27017
     type: RSSecondary
     avg_rtt_ms: 50  # Too far.
     lastUpdateTime: 25002  # Not used when there's no primary.
-    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 25 sec stale + 25 sec heartbeat <= 50 sec maxStaleness.
+    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 125 sec stale + 25 sec heartbeat <= 150 sec maxStaleness.
     maxWireVersion: 5
   - &3
     address: c:27017
@@ -25,7 +25,7 @@ topology_description:
     maxWireVersion: 5
 read_preference:
   mode: Nearest
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
 suitable_servers:
 - *1
 - *2
diff --git a/spec/support/max_staleness/ReplicaSetNoPrimary/Nearest.yml b/spec/support/max_staleness/ReplicaSetNoPrimary/Nearest.yml
index 8474e07198..6f71c92b7d 100644
--- a/spec/support/max_staleness/ReplicaSetNoPrimary/Nearest.yml
+++ b/spec/support/max_staleness/ReplicaSetNoPrimary/Nearest.yml
@@ -7,14 +7,14 @@ topology_description:
     type: RSSecondary
     avg_rtt_ms: 5
     lastUpdateTime: 0
-    lastWrite: {lastWriteDate: {$numberLong: "25002"}}
+    lastWrite: {lastWriteDate: {$numberLong: "125002"}}
     maxWireVersion: 5
   - &2
     address: b:27017
     type: RSSecondary
     avg_rtt_ms: 50  # Too far.
     lastUpdateTime: 0
-    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 25 sec stale + 25 sec heartbeat <= 50 sec maxStaleness.
+    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 125 sec stale + 25 sec heartbeat <= 150 sec maxStaleness.
     maxWireVersion: 5
   - &3
     address: c:27017
@@ -25,7 +25,7 @@ topology_description:
     maxWireVersion: 5
 read_preference:
   mode: Nearest
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
 suitable_servers:
 - *1
 - *2
diff --git a/spec/support/max_staleness/ReplicaSetNoPrimary/Nearest2.yml b/spec/support/max_staleness/ReplicaSetNoPrimary/Nearest2.yml
index 32aaa70fb2..ecf74e7e43 100644
--- a/spec/support/max_staleness/ReplicaSetNoPrimary/Nearest2.yml
+++ b/spec/support/max_staleness/ReplicaSetNoPrimary/Nearest2.yml
@@ -7,14 +7,14 @@ topology_description:
     type: RSSecondary
     avg_rtt_ms: 50  # Too far.
     lastUpdateTime: 0
-    lastWrite: {lastWriteDate: {$numberLong: "25002"}}
+    lastWrite: {lastWriteDate: {$numberLong: "125002"}}
     maxWireVersion: 5
   - &2
     address: b:27017
     type: RSSecondary
     avg_rtt_ms: 5
     lastUpdateTime: 0
-    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 25 sec stale + 25 sec heartbeat <= 50 sec maxStaleness.
+    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 125 sec stale + 25 sec heartbeat <= 150 sec maxStaleness.
     maxWireVersion: 5
   - &3
     address: c:27017
@@ -25,7 +25,7 @@ topology_description:
     maxWireVersion: 5
 read_preference:
   mode: Nearest
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
 suitable_servers:
 - *1
 - *2
diff --git a/spec/support/max_staleness/ReplicaSetNoPrimary/NoKnownServers.yml b/spec/support/max_staleness/ReplicaSetNoPrimary/NoKnownServers.yml
new file mode 100644
index 0000000000..575c367b83
--- /dev/null
+++ b/spec/support/max_staleness/ReplicaSetNoPrimary/NoKnownServers.yml
@@ -0,0 +1,15 @@
+# maxStalenessSeconds must be at least 90 seconds, even with no known servers.
+---
+topology_description:
+  type: ReplicaSetNoPrimary
+  servers:
+  - &1
+    address: a:27017
+    type: Unknown
+  - &2
+    address: b:27017
+    type: Unknown
+read_preference:
+  mode: Nearest
+  maxStalenessSeconds: 1  # Too small.
+error: true
diff --git a/spec/support/max_staleness/ReplicaSetNoPrimary/PrimaryPreferred.yml b/spec/support/max_staleness/ReplicaSetNoPrimary/PrimaryPreferred.yml
index b562adf00d..6260eb85c0 100644
--- a/spec/support/max_staleness/ReplicaSetNoPrimary/PrimaryPreferred.yml
+++ b/spec/support/max_staleness/ReplicaSetNoPrimary/PrimaryPreferred.yml
@@ -20,7 +20,7 @@ topology_description:
     lastWrite: {lastWriteDate: {$numberLong: "1"}}  # Very stale.
 read_preference:
   mode: PrimaryPreferred
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 90
 suitable_servers:
 - *1
 in_latency_window:
diff --git a/spec/support/max_staleness/ReplicaSetNoPrimary/PrimaryPreferred_tags.yml b/spec/support/max_staleness/ReplicaSetNoPrimary/PrimaryPreferred_tags.yml
index 6aea50ab8a..19c297abe8 100644
--- a/spec/support/max_staleness/ReplicaSetNoPrimary/PrimaryPreferred_tags.yml
+++ b/spec/support/max_staleness/ReplicaSetNoPrimary/PrimaryPreferred_tags.yml
@@ -1,4 +1,4 @@
-# maxStalenessMS is applied before tag sets. With tag sets
+# maxStalenessSeconds is applied before tag sets. With tag sets
 # [{data_center: nyc}, {data_center: tokyo}], if the only node in NYC is stale
 # then use Tokyo.
 ---
@@ -11,7 +11,7 @@ topology_description:
     type: RSSecondary
     avg_rtt_ms: 5
     lastUpdateTime: 0
-    lastWrite: {lastWriteDate: {$numberLong: "25002"}}
+    lastWrite: {lastWriteDate: {$numberLong: "125002"}}
     maxWireVersion: 5
     tags:
       data_center: tokyo  # Matches second tag set.
@@ -26,7 +26,7 @@ topology_description:
       data_center: nyc
 read_preference:
   mode: PrimaryPreferred
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
   tag_sets:
   - data_center: nyc
   - data_center: tokyo
diff --git a/spec/support/max_staleness/ReplicaSetNoPrimary/Secondary.yml b/spec/support/max_staleness/ReplicaSetNoPrimary/Secondary.yml
index ebac70a91e..3d6832ff42 100644
--- a/spec/support/max_staleness/ReplicaSetNoPrimary/Secondary.yml
+++ b/spec/support/max_staleness/ReplicaSetNoPrimary/Secondary.yml
@@ -10,7 +10,7 @@ topology_description:
     avg_rtt_ms: 5
     lastUpdateTime: 0
     maxWireVersion: 5
-    lastWrite: {lastWriteDate: {$numberLong: "25002"}}
+    lastWrite: {lastWriteDate: {$numberLong: "125002"}}
     tags:
       data_center: tokyo  # No match, but its lastWriteDate is used in estimate.
   - &2
@@ -19,7 +19,7 @@ topology_description:
     avg_rtt_ms: 5
     lastUpdateTime: 0
     maxWireVersion: 5
-    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 25 sec stale + 25 sec heartbeat <= 50 sec maxStaleness.
+    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 125 sec stale + 25 sec heartbeat <= 150 sec maxStaleness.
     tags:
       data_center: nyc
   - &3
@@ -42,7 +42,7 @@ topology_description:
       data_center: tokyo  # No match.
 read_preference:
   mode: Secondary
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
   tag_sets:
   - data_center: nyc
 suitable_servers:
diff --git a/spec/support/max_staleness/ReplicaSetNoPrimary/SecondaryPreferred.yml b/spec/support/max_staleness/ReplicaSetNoPrimary/SecondaryPreferred.yml
index 6189bf32a3..4c6a49d0cb 100644
--- a/spec/support/max_staleness/ReplicaSetNoPrimary/SecondaryPreferred.yml
+++ b/spec/support/max_staleness/ReplicaSetNoPrimary/SecondaryPreferred.yml
@@ -19,7 +19,7 @@ topology_description:
     lastWrite: {lastWriteDate: {$numberLong: "1"}}  # Very stale.
 read_preference:
   mode: SecondaryPreferred
-  maxStalenessMS: 120000
+  maxStalenessSeconds: 120
 suitable_servers:
 - *1
 in_latency_window:
diff --git a/spec/support/max_staleness/ReplicaSetNoPrimary/SecondaryPreferred_tags.yml b/spec/support/max_staleness/ReplicaSetNoPrimary/SecondaryPreferred_tags.yml
index 427a8bb275..f0e26742f2 100644
--- a/spec/support/max_staleness/ReplicaSetNoPrimary/SecondaryPreferred_tags.yml
+++ b/spec/support/max_staleness/ReplicaSetNoPrimary/SecondaryPreferred_tags.yml
@@ -10,7 +10,7 @@ topology_description:
     avg_rtt_ms: 5
     lastUpdateTime: 0
     maxWireVersion: 5
-    lastWrite: {lastWriteDate: {$numberLong: "25002"}}
+    lastWrite: {lastWriteDate: {$numberLong: "125002"}}
     tags:
       data_center: tokyo  # No match, but its lastWriteDate is used in estimate.
   - &2
@@ -19,7 +19,7 @@ topology_description:
     avg_rtt_ms: 5
     lastUpdateTime: 0
     maxWireVersion: 5
-    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 25 sec stale + 25 sec heartbeat <= 50 sec maxStaleness.
+    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 125 sec stale + 25 sec heartbeat <= 150 sec maxStaleness.
     tags:
       data_center: nyc
   - &3
@@ -42,7 +42,7 @@ topology_description:
       data_center: tokyo  # No match.
 read_preference:
   mode: SecondaryPreferred
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
   tag_sets:
   - data_center: nyc
 suitable_servers:
diff --git a/spec/support/max_staleness/ReplicaSetNoPrimary/ZeroMaxStaleness.yml b/spec/support/max_staleness/ReplicaSetNoPrimary/ZeroMaxStaleness.yml
new file mode 100644
index 0000000000..59e9536daf
--- /dev/null
+++ b/spec/support/max_staleness/ReplicaSetNoPrimary/ZeroMaxStaleness.yml
@@ -0,0 +1,23 @@
+# maxStalenessSeconds=0 is prohibited.
+---
+topology_description:
+  type: ReplicaSetNoPrimary
+  servers:
+  - &1
+    address: a:27017
+    type: RSSecondary
+    avg_rtt_ms: 5
+    lastUpdateTime: 0
+    maxWireVersion: 5
+    lastWrite: {lastWriteDate: {$numberLong: "2"}}
+  - &2
+    address: b:27017
+    type: RSSecondary
+    avg_rtt_ms: 5
+    lastUpdateTime: 0
+    maxWireVersion: 4  # Incompatible.
+    lastWrite: {lastWriteDate: {$numberLong: "1"}}
+read_preference:
+  mode: Nearest
+  maxStalenessSeconds: 0
+error: true
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/Incompatible.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/Incompatible.yml
index 783181133e..9265526895 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/Incompatible.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/Incompatible.yml
@@ -1,6 +1,6 @@
-# During server selection,
-# clients (drivers or mongos) MUST raise an error if ``maxStalenessMS`` is not zero or null,
-# and any server's ``maxWireVersion`` is less than 5 (`SERVER-23893`_).
+# During server selection, clients (drivers or mongos) MUST raise an error if
+# maxStalenessSeconds is defined and not -1 and any server's ``maxWireVersion``
+# is less than 5 (`SERVER-23893`_).
 ---
 topology_description:
   type: ReplicaSetWithPrimary
@@ -21,5 +21,5 @@ topology_description:
     lastWrite: {lastWriteDate: {$numberLong: "1"}}
 read_preference:
   mode: Nearest
-  maxStalenessMS: 120000
+  maxStalenessSeconds: 120
 error: true
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/LastUpdateTime.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/LastUpdateTime.yml
index 4e6bb6411f..129310744f 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/LastUpdateTime.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/LastUpdateTime.yml
@@ -13,21 +13,21 @@ topology_description:
     address: b:27017
     type: RSSecondary
     avg_rtt_ms: 5
-    lastUpdateTime: 25001
-    # Updated 25 sec after primary, so 25 sec stale.
-    # 25 sec stale + 25 sec heartbeat <= 50 sec maxStaleness.
+    lastUpdateTime: 125001
+    # Updated 125 sec after primary, so 125 sec stale.
+    # 125 sec stale + 25 sec heartbeat <= 150 sec maxStaleness.
     lastWrite: {lastWriteDate: {$numberLong: "2"}}
     maxWireVersion: 5
   - &3
     address: c:27017
     type: RSSecondary
     avg_rtt_ms: 5
-    lastUpdateTime: 25001
+    lastUpdateTime: 125001
     lastWrite: {lastWriteDate: {$numberLong: "1"}}  # Too stale.
     maxWireVersion: 5
 read_preference:
   mode: Nearest
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
 suitable_servers:
 - *1
 - *2
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/ShortHeartbeartShortMaxStaleness2.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/LongHeartbeat.yml
similarity index 67%
rename from spec/support/max_staleness/ReplicaSetWithPrimary/ShortHeartbeartShortMaxStaleness2.yml
rename to spec/support/max_staleness/ReplicaSetWithPrimary/LongHeartbeat.yml
index 4a803ef22f..455696492d 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/ShortHeartbeartShortMaxStaleness2.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/LongHeartbeat.yml
@@ -1,7 +1,7 @@
-# Users can configure a shorter ``heartbeatFrequencyMS`` than the default to
-# allow a smaller ``maxStalenessMS`` with replica sets.
+# If users configure a longer ``heartbeatFrequencyMS`` than the default,
+# ``maxStalenessSeconds`` might have a larger minimum.
 ---
-heartbeatFrequencyMS: 1000  # 1 second.
+heartbeatFrequencyMS: 120000  # 120 seconds.
 topology_description:
   type: ReplicaSetWithPrimary
   servers:
@@ -21,7 +21,7 @@ topology_description:
     lastWrite: {lastWriteDate: {$numberLong: "1"}}
 read_preference:
   mode: Nearest
-  maxStalenessMS: 2000  # 2 seconds, permitted.
+  maxStalenessSeconds: 130  # OK, must be 120 + 10 = 130 seconds.
 suitable_servers:
 - *1
 - *2
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/ShortHeartbeartShortMaxStaleness.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/LongHeartbeat2.yml
similarity index 57%
rename from spec/support/max_staleness/ReplicaSetWithPrimary/ShortHeartbeartShortMaxStaleness.yml
rename to spec/support/max_staleness/ReplicaSetWithPrimary/LongHeartbeat2.yml
index a49d75873c..3b6b5adc50 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/ShortHeartbeartShortMaxStaleness.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/LongHeartbeat2.yml
@@ -1,14 +1,14 @@
-# Users can configure a shorter ``heartbeatFrequencyMS`` than the default to
-# allow a smaller ``maxStalenessMS`` with replica sets.
+# If users configure a longer ``heartbeatFrequencyMS`` than the default,
+# ``maxStalenessSeconds`` might have a larger minimum.
 ---
-heartbeatFrequencyMS: 1000  # 1 second.
+heartbeatFrequencyMS: 120000  # 120 seconds.
 topology_description:
   type: ReplicaSetWithPrimary
   servers:
   - &1
     address: a:27017
     type: RSPrimary
-    avg_rtt_ms: 50  # Too far.
+    avg_rtt_ms: 5
     lastUpdateTime: 0
     maxWireVersion: 5
     lastWrite: {lastWriteDate: {$numberLong: "1"}}
@@ -21,9 +21,5 @@ topology_description:
     lastWrite: {lastWriteDate: {$numberLong: "1"}}
 read_preference:
   mode: Nearest
-  maxStalenessMS: 2000  # 2 seconds, permitted.
-suitable_servers:
-- *1
-- *2
-in_latency_window:
-- *2
+  maxStalenessSeconds: 129  # Too small, must be 120 + 10 = 130 seconds.
+error: true
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/MaxStalenessTooSmall.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/MaxStalenessTooSmall.yml
index addd0f8a09..2b5126d4f6 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/MaxStalenessTooSmall.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/MaxStalenessTooSmall.yml
@@ -1,7 +1,8 @@
 # A driver MUST raise an error
 # if the TopologyType is ReplicaSetWithPrimary or ReplicaSetNoPrimary
-# and ``maxStalenessMS`` is less than twice the client's `heartbeatFrequencyMS`.
+# and ``maxStalenessSeconds`` is less than 90.
 ---
+heartbeatFrequencyMS: 500
 topology_description:
   type: ReplicaSetWithPrimary
   servers:
@@ -21,5 +22,5 @@ topology_description:
     lastWrite: {lastWriteDate: {$numberLong: "1"}}
 read_preference:
   mode: Nearest
-  maxStalenessMS: 1  # Too small.
+  maxStalenessSeconds: 89  # Too small.
 error: true
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/MaxStalenessWithModePrimary.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/MaxStalenessWithModePrimary.yml
index 4bd0a158a8..1872d3b365 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/MaxStalenessWithModePrimary.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/MaxStalenessWithModePrimary.yml
@@ -1,4 +1,4 @@
-# Drivers MUST raise an error if ``maxStalenessMS`` is not zero or null
+# Drivers MUST raise an error if maxStalenessSeconds is defined and not -1
 # and the ``mode`` field is 'primary'.
 ---
 topology_description:
@@ -19,5 +19,5 @@ topology_description:
     maxWireVersion: 5
     lastWrite: {lastWriteDate: {$numberLong: "1"}}
 read_preference:
-  maxStalenessMS: 120000
+  maxStalenessSeconds: 120
 error: true
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/Nearest.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/Nearest.yml
index b2516efedb..76915f22b5 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/Nearest.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/Nearest.yml
@@ -7,14 +7,14 @@ topology_description:
     type: RSPrimary
     avg_rtt_ms: 5
     lastUpdateTime: 0
-    lastWrite: {lastWriteDate: {$numberLong: "25002"}}
+    lastWrite: {lastWriteDate: {$numberLong: "125002"}}
     maxWireVersion: 5
   - &2
     address: b:27017
     type: RSSecondary
     avg_rtt_ms: 50  # Too far.
     lastUpdateTime: 0
-    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 25 sec stale + 25 sec heartbeat <= 50 sec maxStaleness.
+    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 125 sec stale + 25 sec heartbeat <= 150 sec maxStaleness.
     maxWireVersion: 5
   - &3
     address: c:27017
@@ -25,7 +25,7 @@ topology_description:
     maxWireVersion: 5
 read_preference:
   mode: Nearest
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
 suitable_servers:
 - *1
 - *2
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/Nearest2.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/Nearest2.yml
index 7e1ac10092..3b249d8f3b 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/Nearest2.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/Nearest2.yml
@@ -7,14 +7,14 @@ topology_description:
     type: RSPrimary
     avg_rtt_ms: 50  # Too far.
     lastUpdateTime: 0
-    lastWrite: {lastWriteDate: {$numberLong: "25002"}}
+    lastWrite: {lastWriteDate: {$numberLong: "125002"}}
     maxWireVersion: 5
   - &2
     address: b:27017
     type: RSSecondary
     avg_rtt_ms: 5
     lastUpdateTime: 0
-    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 25 sec stale + 25 sec heartbeat <= 50 sec maxStaleness.
+    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 125 sec stale + 25 sec heartbeat <= 150 sec maxStaleness.
     maxWireVersion: 5
   - &3
     address: c:27017
@@ -25,7 +25,7 @@ topology_description:
     maxWireVersion: 5
 read_preference:
   mode: Nearest
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
 suitable_servers:
 - *1
 - *2
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/Nearest_tags.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/Nearest_tags.yml
index f44505131f..0c2b6e3ea2 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/Nearest_tags.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/Nearest_tags.yml
@@ -1,4 +1,4 @@
-# maxStalenessMS is applied before tag sets. With tag sets
+# maxStalenessSeconds is applied before tag sets. With tag sets
 # [{data_center: nyc}, {data_center: tokyo}], if the only node in NYC is stale
 # then use Tokyo.
 ---
@@ -11,7 +11,7 @@ topology_description:
     type: RSPrimary
     avg_rtt_ms: 5
     lastUpdateTime: 0
-    lastWrite: {lastWriteDate: {$numberLong: "25002"}}
+    lastWrite: {lastWriteDate: {$numberLong: "125002"}}
     maxWireVersion: 5
     tags:
       data_center: tokyo
@@ -26,7 +26,7 @@ topology_description:
       data_center: nyc
 read_preference:
   mode: Nearest
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
   tag_sets:
   - data_center: nyc
   - data_center: tokyo
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/PrimaryPreferred.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/PrimaryPreferred.yml
index 5717109937..8caa31ed8f 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/PrimaryPreferred.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/PrimaryPreferred.yml
@@ -1,4 +1,4 @@
-# Ignore maxStalenessMS is primary is available.
+# Ignore maxStalenessSeconds if primary is available.
 ---
 heartbeatFrequencyMS: 25000  # 25 seconds.
 topology_description:
@@ -20,7 +20,7 @@ topology_description:
     lastWrite: {lastWriteDate: {$numberLong: "1"}}
 read_preference:
   mode: PrimaryPreferred
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
 suitable_servers:
 - *1
 in_latency_window:
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/PrimaryPreferred_incompatible.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/PrimaryPreferred_incompatible.yml
index 554a9d1021..677a52ce9a 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/PrimaryPreferred_incompatible.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/PrimaryPreferred_incompatible.yml
@@ -1,6 +1,6 @@
 # Primary has wire version 5, secondary has 4, read preference primaryPreferred
-# with maxStalenessMS. The client must error, even though it uses primary and
-# never applies maxStalenessMS. Proves that the compatibility check precedes
+# with maxStalenessSeconds. The client must error, even though it uses primary and
+# never applies maxStalenessSeconds. Proves that the compatibility check precedes
 # filtration.
 ---
 topology_description:
@@ -22,6 +22,6 @@ topology_description:
     lastWrite: {lastWriteDate: {$numberLong: "1"}}
 read_preference:
   mode: PrimaryPreferred
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
 error: true
 
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/SecondaryPreferred.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/SecondaryPreferred.yml
index b56e4ac2e0..ac61c3a994 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/SecondaryPreferred.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/SecondaryPreferred.yml
@@ -19,7 +19,7 @@ topology_description:
     lastWrite: {lastWriteDate: {$numberLong: "1"}}  # Very stale.
 read_preference:
   mode: SecondaryPreferred
-  maxStalenessMS: 120000
+  maxStalenessSeconds: 120
 suitable_servers:
 - *1
 in_latency_window:
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/SecondaryPreferred_tags.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/SecondaryPreferred_tags.yml
index 38cf94ecba..c32e8c41af 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/SecondaryPreferred_tags.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/SecondaryPreferred_tags.yml
@@ -10,14 +10,14 @@ topology_description:
     avg_rtt_ms: 5
     lastUpdateTime: 0
     maxWireVersion: 5
-    lastWrite: {lastWriteDate: {$numberLong: "25002"}}
+    lastWrite: {lastWriteDate: {$numberLong: "125002"}}
   - &2
     address: b:27017
     type: RSSecondary
     avg_rtt_ms: 5
     lastUpdateTime: 0
     maxWireVersion: 5
-    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 25 sec stale + 25 sec heartbeat <= 50 sec maxStaleness.
+    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 125 sec stale + 25 sec heartbeat <= 150 sec maxStaleness.
     tags:
       data_center: nyc
   - &3
@@ -49,7 +49,7 @@ topology_description:
       data_center: tokyo  # No match.
 read_preference:
   mode: SecondaryPreferred
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
   tag_sets:
   - data_center: nyc
 suitable_servers:
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/SecondaryPreferred_tags2.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/SecondaryPreferred_tags2.yml
index 1c61a030f1..eea732a8ce 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/SecondaryPreferred_tags2.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/SecondaryPreferred_tags2.yml
@@ -1,4 +1,4 @@
-# maxStalenessMS is applied before tag sets. With tag sets
+# maxStalenessSeconds is applied before tag sets. With tag sets
 # [{data_center: nyc}, {data_center: tokyo}], if the only secondary in NYC is
 # stale then use Tokyo.
 ---
@@ -11,7 +11,7 @@ topology_description:
     type: RSPrimary
     avg_rtt_ms: 5
     lastUpdateTime: 0
-    lastWrite: {lastWriteDate: {$numberLong: "25002"}}
+    lastWrite: {lastWriteDate: {$numberLong: "125002"}}
     maxWireVersion: 5
   - &2
     address: b:27017
@@ -33,7 +33,7 @@ topology_description:
       data_center: nyc
 read_preference:
   mode: SecondaryPreferred
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
   tag_sets:
   - data_center: nyc
   - data_center: tokyo
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/Secondary_tags.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/Secondary_tags.yml
index 27aa4e1757..3f529477a8 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/Secondary_tags.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/Secondary_tags.yml
@@ -10,14 +10,14 @@ topology_description:
     avg_rtt_ms: 5
     lastUpdateTime: 0
     maxWireVersion: 5
-    lastWrite: {lastWriteDate: {$numberLong: "25002"}}
+    lastWrite: {lastWriteDate: {$numberLong: "125002"}}
   - &2
     address: b:27017
     type: RSSecondary
     avg_rtt_ms: 5
     lastUpdateTime: 0
     maxWireVersion: 5
-    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 25 sec stale + 25 sec heartbeat <= 50 sec maxStaleness.
+    lastWrite: {lastWriteDate: {$numberLong: "2"}}  # 125 sec stale + 25 sec heartbeat <= 150 sec maxStaleness.
     tags:
       data_center: nyc
   - &3
@@ -49,7 +49,7 @@ topology_description:
       data_center: tokyo  # No match.
 read_preference:
   mode: Secondary
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
   tag_sets:
   - data_center: nyc
 suitable_servers:
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/Secondary_tags2.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/Secondary_tags2.yml
index 9d46e85769..e15450b58e 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/Secondary_tags2.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/Secondary_tags2.yml
@@ -1,4 +1,4 @@
-# maxStalenessMS is applied before tag sets. With tag sets
+# maxStalenessSeconds is applied before tag sets. With tag sets
 # [{data_center: nyc}, {data_center: tokyo}], if the only secondary in NYC is
 # stale then use Tokyo.
 ---
@@ -11,7 +11,7 @@ topology_description:
     type: RSPrimary
     avg_rtt_ms: 5
     lastUpdateTime: 0
-    lastWrite: {lastWriteDate: {$numberLong: "25002"}}
+    lastWrite: {lastWriteDate: {$numberLong: "125002"}}
     maxWireVersion: 5
   - &2
     address: b:27017
@@ -33,7 +33,7 @@ topology_description:
       data_center: nyc
 read_preference:
   mode: Secondary
-  maxStalenessMS: 50000
+  maxStalenessSeconds: 150
   tag_sets:
   - data_center: nyc
   - data_center: tokyo
diff --git a/spec/support/max_staleness/ReplicaSetWithPrimary/ZeroMaxStaleness.yml b/spec/support/max_staleness/ReplicaSetWithPrimary/ZeroMaxStaleness.yml
index 2a9634f253..99d3a66f35 100644
--- a/spec/support/max_staleness/ReplicaSetWithPrimary/ZeroMaxStaleness.yml
+++ b/spec/support/max_staleness/ReplicaSetWithPrimary/ZeroMaxStaleness.yml
@@ -1,4 +1,4 @@
-# A ``maxStalenessMS`` of zero or null MUST mean "no maximum".
+# maxStalenessSeconds=0 is prohibited.
 ---
 topology_description:
   type: ReplicaSetWithPrimary
@@ -6,22 +6,18 @@ topology_description:
   - &1
     address: a:27017
     type: RSPrimary
-    avg_rtt_ms: 50  # Too far.
+    avg_rtt_ms: 5
     lastUpdateTime: 0
     maxWireVersion: 5
-    lastWrite: {lastWriteDate: {$numberLong: "1000001"}}
+    lastWrite: {lastWriteDate: {$numberLong: "2"}}
   - &2
     address: b:27017
     type: RSSecondary
     avg_rtt_ms: 5
     lastUpdateTime: 0
-    maxWireVersion: 5
-    lastWrite: {lastWriteDate: {$numberLong: "1"}}  # Very stale.
+    maxWireVersion: 4  # Incompatible.
+    lastWrite: {lastWriteDate: {$numberLong: "1"}}
 read_preference:
   mode: Nearest
-  maxStalenessMS: 0  # Means "no max staleness".
-suitable_servers:  # Very stale server is fine.
-- *1
-- *2
-in_latency_window:
-- *2
+  maxStalenessSeconds: 0
+error: true
diff --git a/spec/support/max_staleness/Sharded/Incompatible.yml b/spec/support/max_staleness/Sharded/Incompatible.yml
index 6188c1ebca..7658136271 100644
--- a/spec/support/max_staleness/Sharded/Incompatible.yml
+++ b/spec/support/max_staleness/Sharded/Incompatible.yml
@@ -1,6 +1,6 @@
-# During server selection,
-# clients (drivers or mongos) MUST raise an error if ``maxStalenessMS`` is not zero or null,
-# and any server's ``maxWireVersion`` is less than 5 (`SERVER-23893`_).
+# During server selection, clients (drivers or mongos) MUST raise an error if
+# maxStalenessSeconds is defined and not -1 and any server's ``maxWireVersion``
+# is less than 5 (`SERVER-23893`_).
 ---
 topology_description:
   type: Sharded
@@ -21,5 +21,5 @@ topology_description:
     lastWrite: {lastWriteDate: {$numberLong: "1"}}
 read_preference:
   mode: Nearest
-  maxStalenessMS: 120000
+  maxStalenessSeconds: 120
 error: true
diff --git a/spec/support/max_staleness/Sharded/SmallMaxStaleness.yml b/spec/support/max_staleness/Sharded/SmallMaxStaleness.yml
index ee8c2b139a..2a3d8e490d 100644
--- a/spec/support/max_staleness/Sharded/SmallMaxStaleness.yml
+++ b/spec/support/max_staleness/Sharded/SmallMaxStaleness.yml
@@ -1,4 +1,4 @@
-# Driver doesn't validate maxStalenessMS for mongos
+# Driver doesn't validate maxStalenessSeconds for mongos
 ---
 heartbeatFrequencyMS: 10000
 topology_description:
@@ -11,10 +11,18 @@ topology_description:
     lastUpdateTime: 0
     maxWireVersion: 5
     lastWrite: {lastWriteDate: {$numberLong: "1"}}
+  - &2
+    address: b:27017
+    type: Mongos
+    avg_rtt_ms: 50  # Too far.
+    lastUpdateTime: 0
+    maxWireVersion: 5
+    lastWrite: {lastWriteDate: {$numberLong: "1"}}
 read_preference:
   mode: Nearest
-  maxStalenessMS: 1  # OK for sharding.
+  maxStalenessSeconds: 1  # OK for sharding.
 suitable_servers:
 - *1
+- *2
 in_latency_window:
 - *1
diff --git a/spec/support/max_staleness/Single/Incompatible.yml b/spec/support/max_staleness/Single/Incompatible.yml
index a6b79b3a14..d85d9714d9 100644
--- a/spec/support/max_staleness/Single/Incompatible.yml
+++ b/spec/support/max_staleness/Single/Incompatible.yml
@@ -1,6 +1,6 @@
-# During server selection,
-# clients (drivers or mongos) MUST raise an error if ``maxStalenessMS`` is not zero or null,
-# and any server's ``maxWireVersion`` is less than 5 (`SERVER-23893`_).
+# During server selection, clients (drivers or mongos) MUST raise an error if
+# maxStalenessSeconds is defined and not -1 and any server's ``maxWireVersion``
+# is less than 5 (`SERVER-23893`_).
 ---
 topology_description:
   type: Single
@@ -14,5 +14,5 @@ topology_description:
     lastWrite: {lastWriteDate: {$numberLong: "1"}}
 read_preference:
   mode: Nearest
-  maxStalenessMS: 120000
+  maxStalenessSeconds: 120
 error: true
diff --git a/spec/support/max_staleness/Single/SmallMaxStaleness.yml b/spec/support/max_staleness/Single/SmallMaxStaleness.yml
index 74a7ba25f3..c4285092b2 100644
--- a/spec/support/max_staleness/Single/SmallMaxStaleness.yml
+++ b/spec/support/max_staleness/Single/SmallMaxStaleness.yml
@@ -1,4 +1,4 @@
-# Driver doesn't validate maxStalenessMS for direct connection.
+# Driver doesn't validate maxStalenessSeconds for direct connection.
 ---
 heartbeatFrequencyMS: 10000
 topology_description:
@@ -13,7 +13,7 @@ topology_description:
     lastWrite: {lastWriteDate: {$numberLong: "1"}}
 read_preference:
   mode: Nearest
-  maxStalenessMS: 1
+  maxStalenessSeconds: 1
 suitable_servers:
 - *1
 in_latency_window:
diff --git a/spec/support/max_staleness/Unknown/SmallMaxStaleness.yml b/spec/support/max_staleness/Unknown/SmallMaxStaleness.yml
new file mode 100644
index 0000000000..7daaa1fb4a
--- /dev/null
+++ b/spec/support/max_staleness/Unknown/SmallMaxStaleness.yml
@@ -0,0 +1,14 @@
+# Driver doesn't validate maxStalenessSeconds while TopologyType is Unknown.
+---
+heartbeatFrequencyMS: 10000
+topology_description:
+  type: Unknown
+  servers:
+  - &1
+    address: a:27017
+    type: Unknown
+read_preference:
+  mode: Nearest
+  maxStalenessSeconds: 1
+suitable_servers: []
+in_latency_window: []
diff --git a/spec/support/server_selection.rb b/spec/support/server_selection.rb
index 4a16ee6c70..b078f68a1c 100644
--- a/spec/support/server_selection.rb
+++ b/spec/support/server_selection.rb
@@ -49,11 +49,6 @@ class Spec
         # @since 2.4.0
         attr_reader :max_staleness
 
-        # @return [ Array<Hash> ] candidate_servers The candidate servers.
-        #
-        # @since 2.0.0
-        attr_reader :candidate_servers
-
         # @return [ Array<Hash> ] eligible_servers The eligible servers before the latency
         #   window is taken into account.
         #
@@ -84,13 +79,13 @@ def initialize(file)
           @test = YAML.load(ERB.new(file.read).result)
           file.close
           @description = "#{@test['topology_description']['type']}: #{File.basename(file)}"
-          @heartbeat_frequency = @test['heartbeatFrequencyMS']
+          @heartbeat_frequency = @test['heartbeatFrequencyMS'] / 1000 if @test['heartbeatFrequencyMS']
           @read_preference = @test['read_preference']
           @read_preference['mode'] = READ_PREFERENCES[@read_preference['mode']]
-          @max_staleness = @read_preference['maxStalenessMS']
+          @max_staleness = @read_preference['maxStalenessSeconds']
           @candidate_servers = @test['topology_description']['servers']
-          @suitable_servers = @test['suitable_servers']
-          @in_latency_window = @test['in_latency_window']
+          @suitable_servers = @test['suitable_servers'] || []
+          @in_latency_window = @test['in_latency_window'] || []
           @type = TOPOLOGY_TYPES[@test['topology_description']['type']]
         end
 
@@ -127,8 +122,7 @@ def server_available?
         #
         # @since 2.4.0
         def invalid_max_staleness?
-          @test['error'] ||
-            candidate_servers.any? { |server| server['maxWireVersion'] < 5 }
+          @test['error']
         end
 
         # The subset of suitable servers that falls within the allowable latency
@@ -149,6 +143,15 @@ def in_latency_window
           @in_latency_window
         end
 
+        # The servers a topology would return as candidates for selection.
+        #
+        # @return [ Array<Hash> ] candidate_servers The candidate servers.
+        #
+        # @since 2.0.0
+        def candidate_servers
+          @candidate_servers.select { |s| s['type'] != 'Unknown' }
+        end
+
         private
 
         def primary