Fredbird

I am getting a bit of chatter in the diff. during heavy accelleration.
From what I'm reading I understand a common cause for this is old/worn out lube/limited slip additive.

My question is what is the proper ratio of additive to Gear Lube?

I've purchased the ACDelco Fully Synthetic and plan to completely drain the diff. and re-fill with the new oil.

c4cruiser

You need one bottle of the GM limited-slip additive. IIRC, it's a 4 ounce bottle.

A full synthetic gear oil is not a requirement in the C4 diffs. A good grade of GL-5 rated gear oil will work. I think the factory fill is a non-synthetic 80W-90 gear oil but you should be OK with what you bought.

You will need a pump to get the old gear oil out. There is no drain plug. The same pump can be used to put the new gear oil in. When I replace gear lube in my '87, I will pump in about half of a bottle of the fresh oil then pour the additive in to the bottle and pump the rest in.

After filling, find an empty parking lot and make a few figure-8 turns at low speed, That will get the additive worked into the clutch pack.