mongodump replicaset - 10 hours data loss acceptable - do we need --oplog (oplog.bson) option?
Yes. OPlog is kind of the same as MySQL binary logs as both are used to sync data to secondary and point in time recovery.

You can use mongodump without --oplog option. You will only loose current writes in your backup. As you have a 10 hour window it wouldn't be an issue.

In this scenario I recommend you to use one of your replica set with slaveDelay.