What happened to gasoline in the mid 1970s?

The lead in gas was in the form of tetraethyl lead (as Mattax previously noted). This compound consisted of four (tetra=four) separate ethyl groups attached to a single lead atom. The lead was present to help the non-hardened valve seats in use at the time. The four ethyl groups contributed to the octane rating (anti knock benefit). Ethanol was used as a replacement for tetraethyl lead in an effort to replace the lost octane benefit as well as to provide more oxygen for the combustion process.