Shouldn't the leaded fuel make the engine less prone to detonation and cylinder cracking? Also how do you tune carburetors on an airplane? Do you target the cruising altitude of the aircraft or above that altitude maybe?
You target an appropriately rich mixture at sea level when tuning the carb or adjusting fuel flow for injected engines. The pilot manually leans the mixture as the aircraft climbs to maintain a reasonable fuel/air ratio.
Owners sometimes call that mixture lever the money lever because it governs fuel flow, aka "dollars spent per minute".