As evident from the code at https://github.com/mongodb/mongo/blob/master/src/mongo/db/write_concern.cpp#L58 , when the user tries to set fsync and j options together, the message is shown as "fsync and j options are not used together" which at times could be confusing to the users. (They might interpret the message to use fsync and j together)
Could you please update the message something similar to "fsync and j options must not be used together"? Thank you