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.

Ultra-low-sulfur diesel (ULSD) is diesel fuel with substantially lowered sulfur content. The move to lower sulfur content allows for the application of advanced emissions control technologies that substantially lower the harmful emissions from diesel combustion. ULSD will run in any engine designed for the ASTM D975 diesel fuels.

The EN590 had been introduced along with the European emission standards. With each of its revisions, the EN 590 had been adapted to lower the sulphur content of diesel fuel – since 2007 this is called ultra low sulphur diesel. The standard EN 590 puts diesel fuel into two groups destined for specific climatic environments. For the “temperate” climatic zones the standard defines six classes from A to F. For the “arctic” climatic zones the standard defines five classes from 0 to 4.