2 questions about 8 3/4 sure grip

I run Mobil 1 synthetic gear lube in a Chevy 12 bolt and it is a posi and works fine.Been in there for about 6 years.I have just recently put the same Mobil 1 in two 8 3/4 rears in my 69 Darts ,haven't driven them much but it has not caused any problems. But maybe some more knowledegable people than myself will know more about it. I actually use a lot of Mobil 1 oil in engines, my richmond 6 speed trans in the Chevelle,my personal truck,my wifes hyundai. It is a damn good product.

Bobby