Oil was put on the inside of the rings to prevent them from gaining surface rust before ever being used. As you've probably noticed, the inside of the rings don't have a black protective finish on them. You could leave a very small amount of an oil film on the surface of the inside of the metal rings to prevent rust. You must be sure to fully degrease the synthetic inserts with either acetone or alcohol. The rings grip the inserts, which in turn grip the scope. A little oil between the rings and inserts will not hurt anything, it'll actually help prevent surface rust and help the inserts pivot into place upon tightening the rings. Regardless of oil or not, the ball and socket design will not allow the inserts to slip inside the rings. However, you must make sure that no oil is between the inserts and scope. The last thing you want is a high quality scope to slip in a high quality set of rings.
