20% should work fine, but make sure you have the correct oil content. I don't know how much different car and airplane fuels are, but I do know that my car (an HPI on road) runs lousy with 10% airplane fuel. (.12 size engine) Runs great with "car" fuel, but I don't know if its due to the different nitro content or something else.
You'll probablly want to invest in a different fueling rig for the airplane, a siphon bulb will work but it's a lot neater and quicker using a small hand pump and a modified fuel cap.


Andy