We can and do, if the engines are upgraded with goodies like electronic ignition (yeah, all those old aircraft engines are using fixed timing magnetos). But a l

We can and do, if the engines are upgraded with goodies like electronic ignition (yeah, all those old aircraft engines are using fixed timing magnetos). But a lot of aircraft engines just aren't. And if you're in a certified plane instead of an experimental plane, good luck ever getting it. Basically any engine made by a European engine manufacturer takes "mogas" without incident, and prefers it because lead causes issues. But to the sibling comment's point, those engines also have goodies like single lever power (so no direct mixture or rpm control). That changes the parameters a lot. Edit: to add on, if you're in an experimental plane, you can do all sorts of things to your engine. You can buy aftermarket electronic fuel injection and ignition kits with variable timing, and reclaim a good deal of fuel economy in the process. Most engines that these kits support will then quite happily run along with a fuel like 94UL. Some people have even converted automobile fuel ignition controllers for the purpose (I hear the Ford V8 ECU is pretty popular for this).