Gasoline is also commonly referred to as petrol.
For most of this century, gasoline contained a chemical derived from lead called tetraethyl lead. The presence of lead in automobile fuel helped lubricate valves and helped keep motorists' engines from "knocking."
In the 1970s, public health officials began to recognize that the presence of lead, in paint, in manufactured products, in gasoline, constituted a human health hazard. Regulations were adopted calling for the phase-out of lead. Many older vehicles, however, could operate properly only on leaded motor fuel. For a decade and a half, most gasoline stations offered both leaded and unleaded gasoline. Today, leaded gasoline has largely been phased out of the U.S. market.
See also Refining process.