S Bridgewater, Pumping lead - GA’s dirty little secret, Aerospace, April 2023, p 36

In the automotive Mogas world, lead additives were eliminated 24 years ago, but this has not yet occurred with Avgas. Lead not only reduces spark knock, it also tackles valve seat recession, but it enters the blood stream of residents near airfields. Both Shell and Swift have produced unleaded Avgas but neither has been completely successful, despite a worldwide target set for 2030.