I tried went to buy some GM rear diff oil and additive from a local dealer *cough*MichaelSteadinWalnutCreek,Ca*cough* a few weeks ago - $49/qt for the oil and $30 for the additive! If I didn't know any better I'd say they are trying to get us all to void our warranty
I ended up going across the street to Napa and getting the same spec in Valvoline synthetic for $10/qt.
