An anti-spin or posi differential requires an appropriate dose of special anti-spin or pose additive. Otherwise, you will hear a clanking coming from the rear of the car when turning. With some synthetic gear lubes, the differential performed fine with out the additive.
Mobil 1 in a "C" posi rear currently made the most horrible banging noises you could imagine. This would occur after running on the straight highway and then stopping and making a turn. After a loud "clang" it would loosen up and be OK until you did the same routine again. I added the GM additive and it went away. So my experience is that Mobil 1 needs the GM additive for use in posi rears.
[ Thanks to Bob Handren, Jason Labey for this information ]
[ Thanks to Mark Prince, Bob Valentine, Jeff Zeilmann, Bill Culp, Dave Paulison, Bob Handren, Nick DiGiovanni, Joe Padavano, Luke, Bruce D. Brumm, Bob Barry, Doug Ahern, Scott Kozhill, Robert Lavey for this information ]