The gears in the supercharger are moving a lot faster than those in the differential.Supercharger = Gears.
Gears = Oil
I used gear oil and if someone has anything to say about it i pose this question... Why would gear oil not be sufficient? It its meant for the crazy stress and high speed of a differential, and the only thing different about that and the m90 gears is that 1. Gears are smaller, not sure on speed might actually be slower. 2. Actually have alot less to no stress on them, especially when most driving is at cruise. Ive had no problems using gear oil and upon removal/ dissasembly found no evidence of wear. But if it makes you feel better, use the GM S/C oil.
Mine spins at almost 15,000 RPM at 6,000 RPM on the motor. A differential has much larger gears and rotating pieces, but I don't think it spins quite that fast. Maybe 8,000 RPM max? Based on that, I don't think 75W-90 gear oil would be appropriate for the SC gears. Something like a very good, low weight synthetic may work, but I'd be looking for something that has a low thermal expansion. The turbine oil sounds like a pretty good idea, though!
Well I know a few people that have been running 10w 30 full synthetic in the superchargers for almost a couple years now and haven't had any issues. I've heard it actually works better than the gm s/c oil. Idk, I'm gunna try it, I'll let youguys know what happens.