Leaded fuel and cars with catalytic converters don't mix. As few as two tanks of leaded gasoline can render a catalytic converter useless, meaning your car will run more roughly and pollute more as well. There may even be safety concerns.
U.S. automakers began installing catalytic converters in cars in 1975 as a response to increasingly strict pollution laws. Their growing use coincided with the gradual elimination of leaded fuel in the United States because of lead's toxicity in the environment. Cars with catalytic converters are designed to run strictly on unleaded fuel.
The catalytic converter is a canister filled with a ceramic matrix of tiny air passages the exhaust gases flow through on the way to the muffler and tailpipe. The ceramic surface is coated with a thin layer of catalyst metals, often palladium, platinum, rhodium or gold. When exhaust gases containing carbon monoxide, unburned fuel and nitrogen oxides contact the metals, a chemical reaction happens turning them into carbon dioxide, nitrogen and water.
Burning leaded fuel in your catalytic converter-equipped car causes several problems. The lead in the fuel reacts with the metal catalyst in the converter and destroys its function, your car will cause more pollution. Because your car was designed to use a catalytic converter, it won't run as well when the converter is nonfunctional. Continuing to use leaded fuel can cause the converter to build up excessive heat and even catch fire under certain conditions.