Fuel oil
Fuel oil in the United States is classified into six classes, according to its boiling temperature, composition and purpose. The boiling point, ranging from 175 to 600 °C, and carbon chain length, 20 to 70  atoms, of the fuel increases with number. Viscosity also increases with fuel oil number and the heaviest oil has to be heated to get it to flow. Price usually decreases as the fuel number increases.