Motul Valve Expert is a lead replacement additive for engines that were built to run on leaded petrol and now have no choice but unleaded. In those engines the lead in the fuel deposited a cushioning layer on the valve seats, and without it the exhaust valves can sink progressively into unhardened cast iron seats, a failure known as valve seat recession, which quietly destroys valve clearances and compression. This additive uses a phosphorus-based chemistry to protect the seat faces and keep the valves clean. It is aimed squarely at classic and vintage vehicles that have not had hardened seats fitted. Because phosphorus poisons catalytic converters, do not use it in any vehicle fitted with a catalyst, and follow the stated dose per tank.