I am gonna change out the fluid in my rear diff and I need a few pointers. 1st, for my car does the rear diff require any of the GM additive? I have heard that it does and doesnt. 2nd, should I use synthetic? I think I will use it but I am curious what you guys think. I appreciate the help in advance.

 

Stock open rear end? No additive, and no on synthetic fluid.

If you have a torsen unit, you CAN add the additive though it isn't necessary.

 

Why no on synthetic?

 

The LSD friction modifier "additive" prevents the posi clutches from getting gummed up, which can make the rearend chatter in the turns.
Open diff: use synthetic or conventional oil, but the additive would be a waste of money.
Clutch style posi/LSD: use conventional oil with the additive.
Torsen style LSD (no clutches), use synthetic or conventional oil. The additive technically isn't needed, but GM issued a TSB that recommended using the additive to quiet down or prevent some noise that a small number of owners complained about.
Why not synthetic in the clutch style posi? Because you want the clutches to grip, not slip.

 

Ok cool. Thanks for the help. I went ahead and bought some synthetic to change it out tonight. They take just under 2 quarts right?

 

Right, 2 quarts. And it's up to you, but I don't bother with the bothersome diff cover gasket. I prefer using nothing but the RTV gasket maker.