OIL is the way to go. Grease just lubes the gears. Oil lubes the gears AND offers another tuning option. If you race you can try different thickness of oil and see what works best. If you bash, just go with something thick, like 30,000wt. oil in your diffs will act like a limited slip, so that when one tire has less traction than the one of the other side, it doesn't automatically "diff out" and take all the power and just spin.....