LeAd was used as an octane enhancer, to prevent engine knock. It was also believed to prevent valve seat wear/recession in older cars.

Lead has serious health impacts - it impairs development of brain functions and lower iQ's in children; it increases heart and respiratory diseases in adults among other associated diseases

Leaded fuel

The difference between leaded and unleaded gasoline is leaded gasoline contains lead and unleaded does not. Leaded gasoline is no longer produced. It was produced from the 1920s until the 1970s. Leaded gasoline gave improved protection for engine valves and it improved resistance to detonation.