I purchased a 10 bolt 3.42 limited slip rear end to put in my Trans am and need to change the fluid in it. I didn't see limited slip additive at O'Rileys so I bought 3 quarts of Castrol 80w-90 'limited slip' Which I assume has the additive already in it. A friend at work is now telling me that I really need the special GM additive. Do I?

 

Yes, you need the additive.
I'm pretty sure "limited slip" on the Castrol bottle legally means it's compatible with limited sip differentials, and doesn't contain the additives or properties that will prevent your clutches from chattering.
It's deceptive advertising to entice more people to buy their product that otherwise may choose a different brand.

 

Lucas claims with their stuff you don't but not sure about castrol. I always use the additive no matter what usually the yukon stuff. I am going to try the GM stuff the next time around about $14 at the dealer.

Oriley's usually has this stuff in stock but I have heard mixed things.


http://www.oreillyauto.com/site/c/de...=limited+slip+

 

I've used the generic parts-store additives from AutoZone and AdvanceAuto with regular Valvoline 80w90, and they work just as well as the GM stuff.

 

Just read the label on the gear oil you bought, if it says for limited slip top off only you need the LSD additive. if it says for limited slip differentials it already has the friction modifiers added so you do not need any more additive.

 

Depends on what type of LSD you have. Any clutch type LSD will need the additive, like the Auburns that would've come in the factory POSI cars. Torsen LSDs that came in later cars (99-02) don't need any additives since they don't have clutches

 

If you bought a rear end with the Auburn differential then you may need to use 2 bottles of the GM LSD oil additive. I haven't seen a Auburn that worked smoothly with only one bottle of the additive. You really need to know what differential you have before putting oil in it.

 

Always add the GM Limited Slip (Posi) additive to the differential. Yes, it sometimes takes a couple of bottles to get it right. You cannot skip out on this.

 

GM and ford have the best additive, I haven't had any of the parts store stuff work without a serious over dose of it and then it doesn't last near as long as the GM or Ford stuff.

 

How much of the bottle are you guys adding to the oil? Ive heard it trial and error on amount and you have to listen to what the gears sound like but I have always just added the entire bottle of the GM stuff.

 

No less than one bottle.

Honestly, one bottle alone has always done it for me, whether GM or generic parts-store stuff.

The only exception was a long time ago when I naively refilled a new vehicle (Auburn posi) with Amsoil synthetic. It took 2 bottles of the GM additive to restore acceptable performance to that one.