Well I'm probably going to change my rear end fluid next week. I have some synthetic gear oil and GM additive. My question is do I need to flush out the rear end with anything or just get as much of the old fluid out as I can? I've heard some say use brake cleaner and some say don't use anything. Any ideas?

 

Synthetic fluid is not recommended for clutch-pack differentials. Just FYI.

 

What do you mean? From SLP, the car had synthetic rear end fluid and I believe that it is the factory limited slip rear end and not the optional rear end from SLP.

 

Who told you that? GM puts a blue full synthetic in all rear ends of there cars and trucks. Mine had full synthetic in the rear and was perfectly fine. Just add the additive.

As far as changing the fluid. Just try and get as much of the old oil out as possible. Then close it up and fill it up.

 

Anyone use Royal Purple synthetic? The bottle says it has limited slip additive in it. Do you still need to add the GM limited slip additive?

 

I am pretty sure you don't need slip additive if the bottle already has some in it. GM fluid though, other fluids might not work.

 

Auburn themselves recommend using non-synthetic oil. I see these questions raised frequently and invariably everyone always suggests using synthetic... some may get by using synthetic fine.. it may be a non issue. I'm not an engineer or a mechanic so I can't really say.. what I can do is quote directly from Auburn's own website:

2.) What kind of oil should be used with an Auburn differential?

It is important when installing Auburn differentials that a high quality, non-synthetic 80w90 GL-5 oil, treated with GM or Ford limited-slip additive (also known as friction modifier) be used. Three ounces of additive will treat one quart of oil.


Go and read it for yourselves, you can find it here:
http://www.auburngear.com/aftermarket/faq/#q2

Based on what others have said, the synthetic oil is bad for a the conical clutches used in auburns diff. Now, if you have a torsen gear driven diff. then synthetic is fine.

I say use the cheap dino oil with the genuine gm additive (not that posi-x crap sold at autozone) and change it out every year or two.

My .02

 

I ended up using Casterol 75w-90 GL-5 synthetic oil with 4 oz. of the GM limited slip additive. I havn't had a problem with any of the clutches sticking or chattering. Keep in mind that I don't have the Auburn differential, but just the standard limited slip. Is there really any difference between the 75w-90 and the 80w-90? I know that GM specifies the second one, but I would imagine I would be fine with the first one?

 

Just for an FYI, the stock posi is an auburn. Yes, you are fine with 75w, 80w is not that much thicker, and with the additive it should be ok.

 

DUH! I just had a brain fart. I was thinking about the optional differential that SLP offered. Once again I have had another brain fart and can't remember what that is called either.

 

Torsen differential.

It's ok, we've all been there.

 

Well lets all pray for my rear end in that I don't screw it up with the synthetic!