I recommend you look up the papers on use of unleaded in Lycontinentals published by the EAA about 5 years ago.
Rather to everybody's surprise, they found that the engine reliability improved across the board.
Lead is only needed to protect valve seats designed to need lead. Most of these geriatric aircraft engine designs we're using at the moment were designed against the old unleaded AVGAS that was used in the USA in the 1930s and 1940s. That said some engines do require lead - that's why considerable work was done on this by CAA and others about 2-4 years ago and AN98A published.
G
[This message has been edited by Genghis the Engineer (edited 17 November 2000).]