Actually, it is not very simple. This should be possible with querysuch as :

var users = collection.Find(user => user.LastModifiedAt > user.LastSyncedAt).ToList();

But unfortunetly MongoDriver could not translate this expression. You could either query all Users and filter on the client side:

var users = collection.Find(Builders<User>.Filter.Empty)
                      .ToEnumerable()
                      .Where(user => user.LastModifiedAt > user.LastSyncedAt)
                      .ToList();

Or send json query, because MongoDb itself is able to do it:

var jsonFliter = "{\"$where\" : \"this.LastModifiedAt>this.LastSyncedAt\"}";
var users = collection.Find(new JsonFilterDefinition<User>(jsonFliter))
                      .ToList();

And, yes, you need an Id - Property for your model class, i haven't mentioned it first, because i thought you do have one, just not posted in the question.

There is another way to do it. First lets declare collection:

var collection = Database.GetCollection<BsonDocument>("CollectionName");

Now lets add our project:

var pro = new BsonDocument {
                {"gt1", new BsonDocument {
                    { "$gt", new BsonArray(){ "$LastModifiedAt", "$LastSyncedAt" }
                    }
                } },
                {"Name", true },
                {"LastModifiedAt", true },
                {"LastSyncedAt", true }
                };

Now lets add our filter:

var filter = Builders<BsonDocument>.Filter.Eq("gt1", true);

We'll aggregate our query:

var aggregate = collection.Aggregate(new AggregateOptions { AllowDiskUse = true })
                .Project(pro)
                .Match(filter)

Now our query is ready. We can check our query as follow:

var query=aggregate.ToString();

Lets run our query as follow:

var query=aggregate.ToList();

This with return the required data in list of bson documents.

This solution will work mongo c# driver 3.6 or above. Please comment in case of any confusion. Hopefully i'll able to explain this.