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 (= 104,44 - 126,66 ° C). 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.