When was unleaded gasoline introduced?

[Skiperrrr]

I have a 1967 Olds Cutlass and I am wondering if I need to add a lead substitute to the gas and how often do I need to add it?

 

[doane_nut]

Mid 1970's. You need to add it to every tank of gas. If you drive it only occasionally, probably not needed. If you drive it a lot and have the money, have the valve seats upgraded to work with unleaded gas. I put a lot of miles on an old 65 Chevy, all unleaded gas, never had any issues, but it was just a beater, not a car I cared about.