I just had an Auburn Hi Performance posi unit with new Richmond gears and Moser axles installed in my 8.5. The shop that installed it gave me an extra quart of 80W90 Synthetic gear lube. I asked them if I needed the GM Posi additive and they said NO. After looking at the installation instructions, Auburn recommends 80W90 NON SYNTHETIC lube with friction additive. Which is correct or is this not a big concern. I have not had the car on the street yet but should be motoring this weekend.