regular motor oil, but gear oil would probably be better, being thicker and having the little squirt top on the bottle. I did just this to my 82 SD several years ago trying to diagnose the noise. It did help quiet the noise and I do it periodically (oops, it's overdue) until I get a chance to relace them. I have more play in mine than I'd like, but the car still rides nice, especially with that rebuilt motor! Without the boots, the joints wash out faster, maybe this time I'll use grease on top of gear oil.