has anyone tried Synthetic GEAR oil like mobile1 or the GM Synthetic in there older thirdgen?

I'm afraid the Synthetic oil will leak past the pinion seal, and axle seals in a higher mileage car.

do I need the posi additive with the Synthetic for my 10 bolt?

you hear so many stories about syn motor oil leaking past old seals in a higer mileage motor, I wonder if the rear is the same......

 

I began using Mobil 1 throughout when the car had better than a hundred thousand on the clock. I personally found nothing problematic, though when I swapped in rear PBR's, I found the rear seals were leaking, but that shouldn't come entirely as a surprise with that many miles (shoot, they may have been leaking before using Mobil 1 for all I know). Many of the guys here, including me, use Mobil 1 75-90 with GM posi lube.

JamesC

 

i've switched every car, truck, and tractor i own or owned to synthetic lubes with out problems, usually M1 or amsoil.