I ended up rebuilding everything on the back half of my car (brilliant idea by the way), new control arm bolts torqued correctly, re-lube/grease everything, cleaned everything, etc and while doing it noticed one of my axle seals was leaking. Glad I noticed, but I seem to be making a lot of additional work for myself lately when I should just get the car done. Anyway I called Torsen to get a fluid recommendation and they wouldn’t give me one for liability reasons (that was odd). So I contacted Rehagen Racing which is a shop/race team that fields a fleet of FR500’s. They use fully synthetic Mobil 1 and run 6 hour endurance races with it. I think it’s fair to say they know what they are doing. I had already purchased some Royal Purple 75-90 and asked what they thought about that and without hesitation said run it. And for the person asking about adding friction modifier to the Royal Purple…..read the bottle. It’s already in there.
I would not run synthetic gear lube. It doesn't bind to metal as well as conventional.