D6 is a type of residual fuel, mainly used in power plants and larger ships. The fuel requires preheating before it can be used. It is not suitable for smaller engines or vessels/vehicles where preheating is not possible. D6 is its name in the USA; in other parts of the world, it is known by different names.

Residual refers to the material remaining after the more valuable cuts of crude oil have boiled off. The residue may contain various undesirable impurities, including 2 percent water and 0.5 percent mineral soil.

D6 fuel is also known as Residual Fuel Oil (RFO), by the Navy specification of Bunker C, or by the Pacific Specification of PS-400.

Recent changes in fuel quality regulation now require further refining of D6 in order to remove the sulfur, which leads to a higher cost. Despite this change, D6 remains less useful due to its viscosity, the need for pre-heating before use, and its high content of pollutants, such as sulfur.

Since it requires pre-heating, it cannot be used in small ships, boats, or cars. However, large ships and power plants can still utilize residual fuel oil.