I have a stock 65 289, and I burn the cheapest gas I can buy. However, I do use a lead substitute. I have no proof that it helps to keep my valves from burning, but so far they haven't. A pint of lead substiture lasts for maybe a year, so you don't use that much. Leaded gas has not been around for maybe 25 years, so I doubt seriously that lead on the valves had been built up to the point to where the lead is still there. Afterall, the reason for putting in lead in gas at the time was not to cushion the valve seats, but to add to the octane rating of the gasoline. A lead substitute costs virtually nothing, and cannot hurt. It may even help. Who knows?