The main difference between leaded and unleaded fuel is the additive tetraethyl lead. Leaded gas was commonly used many years ago but it was discovered that lead has undesirable side effects when the lead is released into the air. Leaded gas was found to be harmful to the environment and humanity which resulted in the production of unleaded gas. You won’t find leaded gas at the pump. Nowadays its used for motor racing, heavy equipment, and marine vehicles. It is also known as petrol or gas.