I've used Castrol GTX Oil in my bikes for years! API SG 10W40 never had a problem so far. Ran it in my V-Max Drag Bike for three years. (talk about abuse) and on regular teardown maintainance,everything looked like new inside. This bike got slammed thru the gears (with a air shift) @ 11500RPM. Time after time! No clutch failures,slippage ect. Here's something else that will get your mind going! It was shaft drive and all the racers that really run the Max used a mixture of 90w hypoid in the diff,plus 10-15% STP! I have kicked around the idea of using that formula in my street bikes rear-end,but never have. They claimed that the STP(I hate the gooie stuff,) cut friction by 20% in the diff. I think that if it's in the owners manual,you will be ok. Some things that aren't in the owners manual are ok,and some aren't RD
