Unleaded

2oldman wrote:

MrWizard wrote:
as far as lead in gasoline, i can't remember it was there for protecting valve wear, or there to raise octane rating or both

It was to slow the rate (or maybe temperature) of ignition thereby decreasing engine knock. (pre-ignition). Problem was it was a scourge on our health.

Google probably knows.

It also protected the valves which have since been redesigned with harder surfaces.