[Steve Chryssos]

For any application that includes a combination of gears and frictions (like a diff), it's best to call your favorite manufacturer or dealer for a recommendation. They'll have a database of hardcore customers who have tested differing levels of friction modifiers cross every possible combination of parts.

Or if you're the shy type, follow the spec of a new ZR1 or similar. It's probably synthetic, but has just the right amount of friction modifier for combined street / track day use.