With almost no exceptions, pilots have exactly zero say in what fuel to use. Airplanes are certified to use certain fuels and that's what you have to use. At this point there is no generally approved unleaded replacement for 100-octane avgas (although several are in testing.)

A few engines, mostly low performance ones, have the option to use automobile gas, but even in those cases it's rarely available at airports so it's not really an option either.