Read all these noteworthy comments, but one issue that nobody seems to ever talk about is fuel stability. The octane can be there, sure, but will it be there next month? Avgas is, what, mil spec’d for two years? Auto fuel goes south in a month. Yes, my airplane will run on mogas, but if it sits out the winter, it best be drained! As far as auto engines are concerned, there are three local “experimenters” that had belt reduction auto engines which all have experienced at least one crankshaft failure. Anecdotal. yes, but I won’t try that in my airplane, thank you.