If I have not had too long a taxi, I can wipe the INSIDE of the exhaust
stack on my O-320 with a white handkerchief and get very little
smudging. I've never experienced a rough mag (or sticking valve) in
its 1700 hrs. Spark plugs are cleaned as an annual insp formality.

A solution that might be explored by non-turboed 100LL users is to use
a combination of FADEC and reduce the compression ratio as necessary to
use auto premium, and reduce the gross weight of the aircraft if
necessary to maintain performance.

I have no answer for turboed aircraft except to note that they are used
primarily at higher altitudes where the compression ratio is not a
limiting factor. Intercoolers would be have to be used universally.

Then we would be down to one environmentally tolerable fuel for piston
engines.