Lead used to serve a necessary lubricating function way back in the 1930s-1940s before the invention of Stellite valve seats, and other hardened alloys for valv

Lead used to serve a necessary lubricating function way back in the 1930s-1940s before the invention of Stellite valve seats, and other hardened alloys for valves. Any aircraft engine that has been either built or overhauled in your or my lifespan already has hardened valve seats installed, or it is a fairly minor procedure to replace them as a part of a top-end overhaul, which often does not even require the engine to be removed. A far worse problem than lead fouling, which just electrically shorts a spark plug and results in power loss and annoying vibration, is lead deposit preignition, where hot bits of lead oxide deposits adhered to the piston act as preignition site triggers, resulting in detonation. Despite the violent sounding name, it will typically go unnoticed by the pilot unless they have automated cylinder head temperature monitoring and alerting, which the majority of piston powered aircraft do not have. At cruise power it can destroy an engine within a minute or so. https://resources.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2010-09_destructive-detonation.pdf https://resources.savvyaviation.com/wp-content/uploads/artic...