Amoco, Esso and Atlantic Richfield all introduced unleaded gasoline in 1970. Beginning with the 1971 model year American car manufacturers started introducing engines that could run on unleaded. I assume that meant hardened valve seats. My 1972 Satellite owners manual specifically mentions it could use unleaded gas. In 1975, most cars started using catalytic converters and were required to use unleaded gas. The Honda Civic with the stratified charge engine was one of the few that met pollution requirements without a converter.
Craig