Four Star (Leaded Fuel) contains Tetra-Ethyl Lead. This was introduced in the 1920’s to raise the octane rating of the fuel resulting in higher compression ratios and power outputs. As engines developed their performance increased as well as their operating temperatures. The coating properties of the lead provided a form of lubrication to the valve seats and other hot elements. This stopped the valves from effectively sticking to the seat whilst operating at these high temperatures. As a result of these high temperatures unleaded fuel cannot be used in an engine with ‘soft’ valve seats. Running the engine in this condition will destroy the valve seats and cause them to recess into the cylinder head.