If you build enough AR-15 uppers, you will eventually come across one that just isn’t going to cooperate with the torque specs. Don’t sweat it. You aren’t going to hurt anything if you have to go beyond the torque spec (within reason). Clint Hanson shares some of his experience with getting those last few millimeters of travel on your barrel nut in Ballistic Advantage’s most recent Builder Series video.
Oil is not what you put on threads.