If you follow the oplog resizing directions in the docs (https://docs.mongodb.com/manual/tutorial/change-oplog-size/) for a 3.5.13 mmapv1 replica set, secondaries get into an error state when they are re-added to the replica set :

2017-09-19T14:01:17.567-0400 E REPL     [rsBackgroundSync] sync producer problem: Location40414: BSON field 'OplogEntryBase.op' is missing but a required field

Note : The procedure in the docs works fine on 3.4.9.

Not sure if this is just a change that needs to be made to the docs for 3.6, or a problem from 3.6 changes. milkie suggested I assign this to replication, but feel free to reassign.