To Blue62:

I don't know if you are running Fox engines or not but they are a bit different than your average Chinese - Taiwanese - Japanese Clone engine.

Fox actually says to use ONLY castor oil in their engines but most Fox experts will agree that if you have at least some castor you will be OK. It's usually a good idea to follow directions at least in the beginning.

Castor gives additional protection in the event of a lean run. If you never get a hole in your fuel line or an air leak anywhere then heck you can run 100% synth oil all day long. You could probably knock the oil percentage down to 10% like the car guys do and get some real 3D power! But I sure wouldn't do it!

However many folks, like Fox prefer the extra protection: just in case.