People are going to talk about this all the time, but from what i have read and done my fair part of research av-gas is ment to be burnt at 30k feet. The air is much thinner, where as race-gas is ment to be burnt at a fair altitude. Even though race-gas is more expensive, it over-all in the long run is better for your engine compared to av-gas. (taken from transworld motocross magazine)