I put nothing but Redline 75W-90 hear oil in the rear diff, and Redline MTL in my older BMW's. The diff's call for 80W-90. And yes, the spec was all over the place for some of the trans. They work great with MTL, and I've seen MTL fix ALOT of hard-shifting condistions, as well as gear clash on alot of those older trannies as well. The M6 should have the G280 dogleg in it iirc, which MTL works great in.
I have yet to see any BMW diff that needs bearings, regardless of mileage. Regular practice is to yank one fro the yard, put new fluid in it, and drive it. Unless you're tracking it, rarely is anything more than seals and/or gaskets required for maintenance. I have yet to see a failed BMW diff that hasn't been tracked. The M6 has a large case 3.91.
FWIW - my current diff in my E30 came from a wrecked 318is, it's a small-case 4.10. I don't think it's ever been apart, I put Redline 75W-90 in it. Pinion seal started leaking due to a broken spring on the seal, I yanked that, replaced the seal, put it back together and back in the car, and have yet to have another issue. The car has now seen 2 full weekends at Road Atlanta, as well as a full season of autox, and very, very hard street driving.