D6 is also be known as Residual Fuel Oil and is of high-viscosity. This particular fuel oil requires preheating to 220 - 260 Degrees Fahrenheit. D6 is mostly used for generators. Recent changes in fuel quality regulation now require further refining of the D6 in order to remove the sulfur, which leads to a higher cost. Despite this recent change, D6 is still less useful because of its viscosity as well as that it needs to be pre-heated before it can be used and contains high amounts of pollutants, such as sulfur. Since it requires pre-heating, it cannot be used in small ships or boats or cars. However large ships and power plants can use the residual fuel oil.