I'm currently using 16% Sidewinder in my .21 (MGT engine). I would have liked to go to 20%-25%, but this was all that was availible at the time. I'm suprised how well the engine runs using this fuel, the engine was easily run-in, keeps cool, and will idle forever. I might step up to 20%, when i've used this up, but i don't see the point in reducing engine life by using 30% for only a small increase (relatively speaking) in power.