Good point. And that is what had me worried about the 18%. And the manual says NOT to run straight castor and that a castor/synthetic is"acceptable" and lists Powermaster as one of the brands. That leads me to believe that a straight synthetic is better. But then the 20/20 Powermaster is 20% nitro (I am guessing, as the can does not clearly tell you), and the manual says 10-15% nitro. But then the manual says that "the next best selection is a high quality 2-cycle glow fuel. That's why I am a little confused.