Recently I service the differetial in my car. Seemed straight forward and I thought gee this stuff is $3 a can and Mobil 1 is is twice the price. Thinking Mobil 1 must be twice as good and of course I added the posi additive (Autozone brand). Everything was fine for one day then the next day my rear made a funny noise as I pulled into a parking spot. Then again when I backed out. This continued until I took it to someone explaining the symptoms. They inspected the rear and said everything looked fine and asked if I had someone install gears. I said no and he told me I had a set of 3.42(yea! better than stock 3.23). So he put new fluid in and added some posi additive. The car has not made the noise and everything seems fine now. Just wanted to share this info. I don't know if it was the Mobil 1 or the additive that caused the problem. It was a little scary thinking I was going to have to shell out hundreds for a rebuild but at least I found out I have a set of 3.42 gears.

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1991 5.7L Z28 - gutted air boxes, K&Ns, airfoil, ported plenum, relocated MAT, accel base, accel 300+ ignition, walboro fuel pump, holley AFPR, vette servo ,flowmaster exhaust, and a custom chip. Ported Accel runners waiting to be installed.

 

i've heard of several people having problems when they used aftermarket posi additive. i always use gm brand. i just built a rear end and put mobil 1 in it.

 

I've also had problems with parts store posi additive. Thats one thing where you should always use GM additive only, accept no substitute.