I bought a 4th gen posi rear from a LT1 car and I thought all of them came with the Torsen units. So I went out and bought all the stuff to service the rear, GM modifer, Mobil 1 Synthetic oil and gasket. When I pulled the cover it defenitly wasn't the Torsen and looked just like the Auburn posi with those horozontal springs.

Now I have a aftermarket Auburn Pro Posi in my other rear and I know with the aftermarket units they specifically tell you not to use synthetic gear oil. With that said I went ahead and serviced this other rear anyway, but I have not driven it yet. What I need to know is can the OEM Auburn units use the synthetic fluid?

GM switched to the Torsen units in 98. Before that, they used Auburns.

I've used synthetic fluid (Mobil 1 and Red Line, that I can recall) with the friction modifier additive in several rears with Auburn units, and it worked fine. It is also said that some brands of lube don't require the additive but I can't verify whether they actually need it or not.

The Zexel unit doesn't need the friction modifier at all because it doesn't use clutches.

I ran Red Line for a couple of years with my aftermarket Auburn unit with no problems to the Posi unit. The reson why so9me lubes don't need the additive is they already contain the additive. You have to read the label to be sure....