Motul System Keep Clean Diesel is a 300ml in-tank treatment for diesel vehicles, meant to be used routinely rather than as a rescue for an already fouled system. Modern common rail injectors work at very high pressures through extremely fine nozzles, so even small carbon and lacquer deposits distort the spray pattern, which costs combustion efficiency and shows up as rough running, smoke and poorer consumption. A detergent additive keeps the nozzle tips and combustion chamber surfaces clean so injection stays close to how it was designed. Add it to the tank before filling up so it disperses through the fuel. Use the petrol version in a petrol vehicle, and follow the stated dose for your tank capacity.