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Discussion Starter · #1 ·
When changing out the rear end fluid, what is better to use, synthetic or regular hypoid? Do you need to use the posi additive with Synthetic rear end fluid?

Next question, what is the best fluid brand to use?

Thanks for the help.

powersix
 

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Fuhgeddaboudit
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If you rear is still stock use GM rear limited slip lube and a bottle of additive. If you changed the rear use what they recommend. I know Eaton and Auburn warns against using Synthetic.
 

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i have a aburn pro and the gear kit i got came with red line fluid and aburn said not to use it use reg gear oil with the additive
 

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MASTER GM TECH
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676 Posts
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
:D ever notice the cars in the pits running in gear on jackstands?
headbang
 

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Why have one ?
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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 . :) .
 

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Risky Business
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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.
 

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I've experienced much of the same thing.
Best results have been with regular Dino oil and 1 - 2 bottles/tubes of additive.
I personally prefer the FOMOCO friction modifier to the GM or generic stuff
 

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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.
 

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So easy cavemen can do it
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i use valvoline durablend semi-syn 85w140 gear oil in my 100k stock posi rear with a bottle of gm posi additive...not a prob here
 

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Regular oil as compared to synthetic-oil supposedly made from decomposing dinosaurs altho oil does not come from said dinos.

Try to avoid using two bottles of additive as it tends to create slippage and accelerates clutch replacement.
 

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Discussion Starter · #12 ·
Thnaks Steve, I will go with the regular hydrocarbon oil with one bottle of additive, I will buy it all from the General I guess.....

Thanks again
 

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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!
 

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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! :mad:
 
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