D6 is a type of residual fuel, mainly used in power plants and larger ships. The fuel is required to be preheated before it can be used. It is not possible to use it in smaller engines or vessels/vehicles where it is not possible to pre-heat it. D6 is its name in the USA. In other parts of the world it has other names. Number 6 fuel oil is a high-viscosity residual oil requiring preheating to 220 - 260 F (104 - 127 C).
