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 ]