Led was added to fuel to cushion the valves, IIRC. Manufacturers made harder steel valves and seats eliminating the need. There was a transition period and I clearly recall people swearing that lead-free fuel was bad for their new car. I don't know. Lead was an additive in the form of tetraethyl lead. Burning resulted in aerosol lead. I think that Mexico may still use leaded gasoline, but I haven't bought gas in MX in a looong time.

Sulfur is a terrible pollutant. It combines readily with moisture in the atmosphere to make sulfurous or sulfuric acid--both highly corrosive. They are one of the principle causes of decomposition of limestone and marble edifices. I recall a study of Greek ruins some years ago in which the scientists could actually measure the degradation of the pillars and statues that had occurred since Athenian smog reached some critical frequency.

Also, sulfur changes the pH (acidity/alkalinity) of formerly pristine mountain lakes and stream, which affects the algae community shifting to dominant species that can withstand lower pH. These organisms are often not the preferred species of the organisms that feed on algae, lowering the rate of weight gain of each dependent species. This ultimately depresses the weigh gain of fish. Also, changes in pH affect fish fertility and reproductive success.

Sometimes the changes are quite unexpected as when sulfur enters water that is deficient in sulfur, a required micronutrient of plants and animals. It can result in huge population bursts in formerly sulfur-deficient species.

In very small amounts sulfur usually acts as a fertilizer. In larger amounts it is a definite polluter.

B