heck, I just switched over to 30% race formula for my n-rusty...... it shouldn't harm anything if you run 20% fuel... if you run 10%(since it is almost impossible to find 15, for me atleast) then you will go real slow... 20% is the safe way to go when it comes to fuel, doesn't have too much nitro, and isn't filled with to much oil...

I think you should try 20%, and if you don't like it, you can always switch to a lower %... just remember to take it easy for the first tank if switching fuels.