Studies with reformulated and oxygenated gasolines of moderate properties have shown no more risk of fuel leaks than conventional gasolines. In nearly all cases, unleaded gasoline will cause no problem when used in engines designed for leaded gasoline. Engineering studies show lead in gasoline reduced risk of exhaust valve damage when an engine was operated under unusually stressful conditions. Stressful conditions include prolonged high speed, high temperature driving, high speed trailer towing, or carrying unusually heavy loads. Under these conditions, there is risk of exhaust valve recession.
Extreme recession can lead to engine failure.
If the kind of driving you do with your older vehicle, built to run on leaded gasoline, is stressful, then using a supplemental lead substitute additive will minimize risk of recession. Normal driving does not require supplemental additives. In these cars, adding supplemental valve lubricant additive may damage emission control systems. MMT is not present in FormulaShell gasolines sold in the United States. MMT is an additive recently permitted in conventional gasolines, but not reformulated or California Phase 2 gasoline.