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would plain old grease add extra resistance?
All other things being equal, it would reduce resistance. Of course, all other things are not equal once you add lubrication, notably the same tightening torque will result in higher bolt tension, which may in itself reduce the tendency to come undone spontaneously.
Just get the right grade of thread locking compound for your intended service.
If I want to add some friction to a bolt to stop it coming undone easily, but wanted something lighter than blue threadlock* and didn't want to buy green threadlock, would plain old grease add extra resistance?
Cheers.
*it may be undone and done up regularly