I just changed my diff fluid today. I’d say about a pint(drained out!). I refilled it with some standard Peak diff fluid as suggested by my mechanic, but this evening a friend asked if I had a Posi and did I put additive in too?
Did I miss something, will it cause problems and what should I do to correct things??

 

You have a posi in the 91 Z28 and the fluid you add needs to have the "friction modifier" otherwise the fluid will burn. Some fluid has it already in it, or you can buy the modifier and just squirt it in to the pumpkin- it's a small tube. Check what you put in there. If it says its for limited slip then you should be fine.

 

You should put the additive in it, but you're not gonna kill it right away.You should be fine. How does only a pint come out? The diff should hold more than that i think. Did you remove the cover?

 

You might want to check your RPO codes, look for G80 which is a posi. Then you'd need to check your owner's manual to see what it recommends. Might have a Gov Lok, cone type, or clutch type posi. The clutch type does require the additive. You'll know when it gets good and hot, you go around a corner, and the rear end jumps/thumps.

 

Well, the fluid won't burn, issue is without the lube, the posi will not have enough lubrication, so when you take a turn on a corner, the posi has to "slip"...the additive GM has, reduces when the "slip" occurs.
Think of a chattering clutch.