Rear end gear oil

Normally, you do need to add a Sure-Grip additive. Some gear oils, like Amsoil Severe Gear, already has a limited slip additive in it. But they also sell it separately just in case the clutches need more.

AMSOIL Synthetic Gear Lube - Gear Oil

The lower multi-viscosity gear oils have worked well for me. i.e.: 75w-90, even in my race car.