RE: my girlfriend got me an Evo 50!
The Sig heli fuel I use has 22% synthetic oil. I think the lowest you would want to go would be 20%. Someone with more heli experience may want to chime in on this one. If you don't get an answer, before you try it, go ask in the main forum. You don't want to ruin your engine. They may not look different, but I'm guessing that they are.
-Pat