If you filled your tank prior to the 1990's, you might remember that there were two choices of gasoline: "regular" and "unleaded." At the time, "regular" gasoline contained tetraethyl lead, which served as an octane booster in fuel. All vehicles manufactured after 1975 are built with catalytic converters and require unleaded gasoline. Catalytic converters help convert pollutants in exhaust to less toxic gasses such as carbon dioxide, nitrogen, and water vapor. Due to environmental concerns, leaded gasoline was phased out in the 1990’s and can no longer be sold at any gas station in the United States. Today, "regular gas" no longer refers to gasoline with lead content, but rather to a certain grade of unleaded gasoline.