Snapper, that little dimple right in the middle of the rear of the housing is the grease fitting. There's a tiny ball bearing in there and you use a needle nosed grease gun to squirt the grease right into it. That greases the rear bearing and the rotor shaft is hollow so that grease goes on through it to get the front bearing, the hammers and hammer frame. You can buy a small grease gun from the tool trucks, some auto parts stores, etc., and I've seen them at Northern Tool, but couldn't find one on their web site just now. You can also buy a needle nosed adapter to put on your regular grease gun. Of course, Ingersoll-Rand recommends using their grease (and I did), but a lot of mechanics just use whatever grease they have in their grease gun. Anything's better than no grease at all. /forums/images/graemlins/laugh.gif Incidentally, if you overgrease it, you will reduce the power until the surplus works its way out, but it won't damage or hurt the tool. Taking the hammer case off the front end to grease it will work, too, of course except that none gets to the rear bearing unless you shoot grease into the front end of the rotor shaft and push it backwards through that hollow shaft.
No oil used in the IR231 except the air tool oil into the air intake. For the models with the screw on the side to put oil in, the recommended oil is 20W or 30W non-detergent motor oil, and then those models require no greasing.