I've now used Synthetic GL-5 75w-90 with no additive, Synthetic GL-5 75w-90 + GM Additive and GM 80W90 GL-4 + GM Additive. I tried them in that order over my last 3 differential changes.

I don't really recall how the 75w90 performed as i wasnt really watching for it closely at the time. It did seem fine though, no problems that come to mind.

I found that Synthetic with the GM additive was the poorest performing in terms of the limited slip functioning properly. It would seem to "slip" too soon on turns under harder accelleration causing the car to feel unstable or swervey. I'd notice it doing this while flooring it onto the highway from an onramp (and switching lanes). Other than the clutches appearing to slip too soon, the Differential behaved fine in all other cases. Also there appeared to be very little wear (metal shavings on the magnet) inside when i changed the fluid out

I now have the GM 80W90 GL-4 + GM Additive and it is definatly behaving much better when acellerating hard while in turns or acellerating onto the highway. The slip I had before is 100% gone.

So from my experience:
Synthetic GL-5 75w-90 with no additive - Probably Good
Synthetic GL-5 75w-90 + GM Additive - Bad for POSI. Good for Wear, seems to be too slippery.
GM GL-4 80W90 + GM Additive - Good for POSI, will find out about wear on next change

So take it for what its worth, these are just my opinions based on how i've felt them perform.

 

any updates? Mine is due.

 

If you were to ask the following 4 departments in a company (sales, marketing, engineering, warranty) rather synthetic or mineral based oil is better for your axle/differential you would get the following responses:

Sales: Synthetic
Marketing: Synthetic
Engineering: Mineral
Warranty: Mineral

Who you would choose to listen to is entirely up to you.

 

I'm curious why GL-4 (moderate duty) was even used, when the factory recommends GL-5 (moderate to severe duty, with double the extreme pressure additives).

It's also unclear what the term "the clutches appearing to slip too soon" means. The clutches always slip in a turn, and lock only when the torque load is high, and the wheels start to turn at different speeds. It would appear the phenomenon being described is erratic locking and unlocking of the clutches, resulting in tire slip.

 

Yeah his description sounds like wheel spin to me too. When you push the loud pedal and the car gets squirely it's the diff working right.

 

if that's your stock diff, it's the auburn and it's probably shot but you didn't mention mileage. the LS1 cars have torsen diffs that don't have wear parts like clutches

i have an LS1 rear and running DuraBlend 80W-90 and it's working well. also ran redline heavy and light shockproof before. soon i'm going to an auburn race diff w/ carbon clutches for autox

 

You guys seem tho change rear end fluid about as often as I change my oil!

Rich

 

According to the owners manual a clutch type Auburn in a 93-98 needs to have the oil changed every 15k miles.

 

My 94 manual indicates only one recommended change at 7,500 miles, with no changes after that. There is no 15,000 mile interval shown.

 

That's odd that they would change that for the 98 model year with the same rear diff. I ***-u-me d that all the clutch Auburn cars would have required the same.