So in the specific case of Cessnas, the first Cessna 172 to require leaded fuel was built in 1977. That was the year they switched from a 6 cylinder Continental O-300 engine to that goddamn 4 cylinder Lycoming O-320 H2AD engine. Bigger displacement, two fewer cylinders, more compression, required leaded fuel.

I gave flight instruction in a new built light sport aircraft equipped with a Rotax 912 engine. Instead of Lycoming and Continental's 1930's era tech, BRP-Rotax's aircraft engines are basically 1980's motorcycle engines. Water cooled, Nikasil cylinder linings, electronic ignition, constant velocity carburetors or EFI, gear reduction, and they run absolutely fantastic on regular automotive gasoline. No lead.