Fuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residue. Broadly speaking fuel oil is any liquid petroleum product that is burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power, except oils having a flash point of approximately 40 °C (104 °F). Fuel oil is made of longhydrocarbon chains, particularly alkanes, cycloalkanes and aromatics. different organizations may have different numerical specifications for the six fuel oil grades. The boiling point and carbon chain length of the fuel increases with fuel oil number. Viscosity also increases with number.