I would think any good GL5 spec 80w140 fully synthetic oil should be good. There are no parts for the sulphur to attack. I actually think it's a Hypoid not a bevel so the shear is even higher. I just switched to redline 80w140 GL5. The knocking when you go over a bump on light load is the backlash in the bevel gear and is normal. As the swing arm comes up it unloads the output of the box as the two share the same centre point and then it clanks when the load comes back on. Good luck
