Most petroleum products can be used as fuel, but the term "fuel oil" varies by region. In Europe, the concept of fuel oil generally refers to the black, thick residue left after distillation, or its mixture with lighter fractions, primarily used as fuel for steam boilers and various heating furnaces, as well as for large, slow-speed diesel fuel and as industrial fuels.

Boiler Fuel Manufacturers Advise: Fuel oil is a type of refined oil, widely used in power plant electricity generation, ship boiler fuel, heating furnace fuel, metallurgical furnaces, and other industrial furnace fuels. Fuel oil is primarily made from cracked residue oil and straight-run residue oil from crude oil, characterized by its high viscosity and high content of non-hydrocarbon compounds, resins, and asphaltene.