My 390 with 129,000 never-rebuilt miles runs fine on premium unleaded. I add Marvel Mystery Oil to every tankful just to help compression and keep things from getting gummed-up. If you want to improve octane, why not add real lead to your gas instead of a lead substitute. Old-fashioned tetra-ethyl lead was outlawed for autos, but not for boats and marine use. You can buy it by the quart at many marinas or through Auto Krafters at
www.autokrafters.com. Good luck.