I own a four-cylinder Lycoming that was run exclusively on an auto fuel STC for 20 years. That's not an engineering problem, its a paper problem. If the aircraft was on N-register in Europe, I'd be running auto fuel in it today.
(however I hate the smell of the stuff)
No need to have it on N-reg in Europe. I'm running my 150(with STC) on mogas whenever I can.