I have a 9mm AR and am replacing the hand guard. The new one came with a barrel nut that is round but that has a faceted section for a wrench (like a crescent wrench, not barrel nut wrench). Old barrel nut comes off easily and new one will screw right on. Do I have to use a torque wrench on the barrel nut or am I safe to just tighten it up? Also, when I removed the barrel it was very oily (the part that slides into the upper) - should it be?