For street use: 75W90 gear oil plus GM's friction modifier. The friction modifier makes the lube more slippery. This allows the clutches to let go more easily, and last longer. It also keeps the rear quieter. (Note: Redline's 75W90 doesn't require the GM modifier, they claim.) For race use: straight 75W90 gear oil with no GM additive. The posi will grab better but make lots of noise and possibly not last as long. (Note: Redline recommends their 75W90NS in this case, since their 75W90 is "slippery" enough to act as if it has the additive).