What happened to gasoline in the mid 1970s?

DFX 340 Duster said
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.

According to Chevron's pdf book, while ethanol may be used to boost octane, that was not the initial purpose of its introduction.
As of 2009

The Super Premium my family generally used was American.
American (owned by Standard Oil of Indiana) never used lead in their highest octane motor gasoline.
"While most oil companies were switching to leaded gasolines en masse during the mid-to-late 1920s, American Oil chose to continue marketing its premium-grade "Amoco-Gas" (later Amoco Super-Premium) as a lead-free gasoline by using aromatics rather than tetraethyl lead to increase octane levels - decades before the environmental movement of the early 1970s led to more stringent auto emission controls which ultimately mandated the universal phase out of leaded gasoline. The "Amoco" lead-free gasoline was sold at American's stations in the eastern and southern U.S. alongside American Regular gasoline, which was a leaded fuel."
from AMOCO | Petroleum Equipment Institute

saved from https://www.historicvehicle.org/vanishing-americana-roadside-business-edition/

Why methanol is not used is touched on in this sidebar on page 55 of the Chevron book