It hasn't been a regulation issue from the FAA that piston engines must use leaded fuel for some reason but rather there hasn't been a viable unleaded aviation fuel that worked with all piston engines.

This is about to change though with G100UL finally being approved. https://gami.com/g100ul/g100ul.php.

It’s on the type certificate that stock Lycoming and Continental engines are certified to run on 100LL. The standard for 100LL is a composition standard (not a performance standard; it literally has to have x grams of TEL/gallon, not just have a certain demonstrated octane).

Absent an STC (Supplemental Type Certificate) to change that, there is a regulation that those engines in certified aircraft have to use leaded fuel to be airworthy (legal) as airworthiness requires both “in condition for safe operation” and “in conformance with its type design”.

GAMI is pursuing an all-model (or many, many model) STC for their fuel to remedy this, but it’s a regulation issue currently.

I would, as I did, call regulations forcing stupidity as a lack of meaningful regulation; but I take your point. It is even a problem of negative regulation.

Small jets do indeed use jet fuel but have a terrible efficiency proposition when ran at even 50% capacity, which is not the standard use case.

And some engines are rated to run automotive gas, but it's just not available at smaller airports.

Most of those are only rated to run on E0 (ethanol-free) gasoline, which is also hard to find outside the airport fence in most places.

I was actually surprised when I looked how many places carry e0. There were several near me in Seattle. https://www.pure-gas.org/

Off-topic question. Did you have to advance your ignition timing when you started running E0? I've thought about using it in my Datsun, especially with E15 rolling out now. I've got a 1980 280ZX, modded with flat top pistons mated w/ a p79 head for about 10:1 compression. Currently though, with normal E10 92 octane gas, I have the distributor set to maximum advance and even with that I feel like it's not getting peak performance. In theory I should already be running lower octane for more advance, but I don't b/c I worry about detonation, and my understanding is that E0 is also more prone to detonation than the same octane ethanol blend b/c it runs hotter without the evaporative cooling of the ethanol, so I might even need to back off the advance when really I'd like to increase it. Did you find running E0 retarded the timing and/or led to any performance changes?

I’ve run some modded engines (including higher than stock compression) and for street usage, I’d be worried about the variability of gas you get over the years. You’re eventually going to get some crap gas on a hot day and have a bad day if you’re right on the ragged edge (especially without knock sensing).

For track or drag usage, running on the ragged edge makes more sense.

I use it for small engines and my motorcycle for the most part. The place by me is all boats, performance cars, snowmobiles, and people with 7 Jerry cans.