What happened to gasoline in the mid 1970s?

What I remember, around the middle of 1985 the amount of lead in regular gas was reduced. My 440 that I'd built to run on Shell regular suddenly started pinging. At the time, many were mixing unleaded premium with leaded regular. The resulting octane was higher than the unmixed premium. The experts at the time claimed because the regular had so much extra lead in it, that boosted the octane of the premium. Now after the lead was reduced, this trick didn't work any longer. Now this was in Louisiana, have no idea what was going on elsewhere.