Yes - for my Wavetrac limited-slip, only Motul Gear 300!
For standard / stock differential, best to use non-synthetic... I've had leakage problems when switching to synthetic. The factory specified rear cover sealant is standard Hylomar and it synthetic will leak through that stuff. If you remove the diff, replace the side seals, and use different sealant on the rear cover - then yup, no problems using synthetic gear oil with the stock diff.