D6 fuel oil is the least useful of all the fuels because it�s a high-viscosity residual oil requiring preheating to 220 � 260 �F (104 � 127 �C). Residual means 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 one-half percent mineral soil. This fuel has other names and may be known as residual fuel oil (RFO), by the Navy specification of Bunker C, or by the Pacific Specification of PS-400.

It is a relatively cheap fuel because of the undesirable properties, but still is used in some industrial boilers, power plants and large ships that can accommodate the heating equipment required to heat up the fuel.