Hour and a half long conversation about trying to standardize unleaded airplane fuel:
https://www.youtube.com/watch?v=0CH2m7PxnLAThey're going about it by testing each airplane and each engine, and then selling people the paperwork that says your plane is compatible. My first reaction is if they can't get some congressional force-of-law thing written that says "this fuel is compatible with everything" then leaded fuel is not going anywhere.
My second reaction is: DAMN fuel is complicated. Avgas is 6-9$/gallon, and all of that money is just going to the fact that they need a special refinery line. The specification of avgas is strict enough that you can't just add lead to car gas and call it a day.
If you're gonna buy an engine, you'd better buy one that can burn the cheapest most standardized most widely available crap that's out there. And when you zoom out a little more, it's clear that electricity is just so much easier to deal with than liquid fuels - you shouldn't be using fuels at all unless you absolutely need to...