Fuel oil is a low-grade heavy fuel oil used in generating plants and similar applications. In the United States and Western Europe, fuel oil is blended or broken, with the final product being diesel.

Fuel oil can be used to heat houses in the former USSR and in the countries of the Far East, which do not have the means to mix or decompose it into more traditional petrochemical products. In the West, stoves that burn fuel oil are commonly referred to as "waste oil" or "waste oil."

Fuel oil-100 is fuel oil, which is made according to GOST, for example GOST 10585-75, GOST 10585-99 Oil fuel. Mazut. Specifications (active, recent changes 07.01.2010). (GOST - the Russian system of standards, for example, ASTM or ANSI). Fuel oil is almost exclusively produced in the Russian Federation, Kazakhstan, Azerbaijan and Turkmenistan. This product is usually used for large boilers in steam production, since the BTU content is high. The most important consideration (and not the only consideration) when sorting this fuel is the sulfur content, which is mainly influenced by the feedstock. For shipment purposes, this product is considered “dirty oil”, and because viscosity drastically affects the ability to pump, delivery has unique requirements. Fuel oil is very similar to oil No. 6 and is part of the products left after gasoline and lighter components evaporate from crude oil.