yup stick with gm lude and additives...
synth may mess with friction properties of posi
besides a warm rearend will turn as easy as a synth lubed 1
ever notice the cars in the pits running in gear on jackstands?
used syn rear end fluid once with GM additive . rear end chattered like crazy when turning corners . dumped it out right away , and replaced it with dino gear oil and GM additive . noise disappered . .
I'm using regular dino 80w-90 (Pennzoil) in the rear right now. I first added one bottle of GM posi addative, and it was fine for a couple months or so. Then it started to chatter a little. I added another bottle of posi addative and it went away. Havent heard anything since.
Original owner told me he always had to use two of the 4 oz bottles to eliminate chatter.
I wouldnt bother with synthetic in a stock rear end. Only if you've got something special and synth is specifically recommended.
I put Mobil 1 75w90 synthetic in my stocker a year ago (or about 2500 miles ago) and a bottle of the GM posi additive and I have had no trouble at all. My father did the same in his vette and it chattered a litte so he had to add a bit more of the additive which got rid of the noise.
My local rear-end rebuilder says not to EVER USE STNTHETICS in an rear end. He only uses GM Posi Additive in all his rebuilds with either an Auburn or Eaton posi and uses two bottles per fill. I always use two bottles in my rear fill and the Buick Zone Rep. concured. I just lost my nice stock 13K mile rear end using that "Amsoil" CRAP Synthetic Gear Lube. Gene (the gear noise is almost deafening even at a very low speed) PS. I'm suing Amsoil because of this damage!
I used amsoil seies 2000 gear oil at there recommendation inthe strange 12 bolt in my 96 ta. Smoked the posi after three passes (11.41 128).Now have to change clutches and springs in a new rear. amsoil sucks!