Have there been any publicized studies done on the reliability of aircraft engines, with and without lead? I’m guessing this is what made the transition take so long, but I can’t seem to find any references.
It’s a rather complex topic with all sorts of factors including economics of a small market and FAA red tape. Av web has a great multi part on failure modes of engines and why new engines often don’t succeed
Another problem is, IIRC, that many replacements for leaded fuels have evaporation & performance issues at lower pressures that don't matter in a car but can be critical in airplane (it's even worse with ethanol additives, IIRC).
There are some claims out there that engines run better without lead (it eats away at seals and whatnot).
Also fouls spark plugs, sticks valves, contaminates oil, and covers the underside of the plane. Leaded avgas is all-around nasty stuff.
I can't find it, but NASA concluded that piston engine reliability is generally a mystery.