When I was trying to sell my '01 Corvette, the diff was making a whine that scared away one potential buyer. I posted a question on Corvette Forum and a member told me to put some limited slip additive in the differential. The GM part number is 1052358. I went to a local dealer, got the little four-ounce bottle from the parts department and squeezed some into the filler hole on the diff. No more whine.
I squeezed in the whole four ounces, but about half of that ran back out again. My vast math skills leave me to believe, therefore, that two ounces should've been about right.