No, this is a myth. In reality, lead fouling decreases engine performance and longevity. From https://www.shell.com/business-customers/aviation/aeroshell/know

No, this is a myth. In reality, lead fouling decreases engine performance and longevity. From https://www.shell.com/business-customers/aviation/aeroshell/knowledge-centre/technical-talk/techart-18-30071600.html https://www.shell.com/business-customers/aviation/aeroshell/...: "The temperature for Lead deposits to form tend to be favourable around the spark plugs (as the whole mixture is quite cool before the flame starts to propagate) and on the exhaust valve stem (as the mixture cools after combustion). The problem is that the deposits are electrically conductive, which shorts out the spark plug - and corrosive, which can start to attack the metal of the valve stems."