i got my drivers license in New Jersey in 1964. At that time, stations either had two or three grades of gasoline. All, except for Amoco premium, had tetraethylead, (TEL) so, technically, they were all "ethyl".
By 1971, I was an engineer working at a Exxon oil refinery. We made three grades of gasoline, regular, "plus", and premium. Environmentally sensitive consumers began wanting a gasoline with less lead content. So Exxon decided to make the Plus grade a 'low lead" blend. However, this was all smoke and mirrors, as it actually resulted in more lead entering the atmosphere from tailpipes.
Why was that? Because TEL in gasoline was the most expensive component. The first unit of volume added gave a good octane boost. But the second yielded a smaller octane boost, and the third unit even less. A 'diminishing return" situation. And consumers using regular and premium started to notice a greater incidence of lead fouling of spark plugs.
this was because, to lower the lead in the "plus" gasoline, better octane distillates had to be diverted from the premium and regular blends. To bring those blends back up to the octane spec, the amount of TEL in those blends had to be approximately doubled.
As a result, to blend all three grades, including the low lead grade, the total TEL we blended into gasoline went up by about 35%. The TEL came in by railroad car. TEL was made by Ethyl Corp and DuPont