What is another word for residual oil?
Pronunciation: [ɹɪsˈɪd͡ʒuːə͡l ˈɔ͡ɪl] (IPA) 
Residual oil is a heavy petroleum product left over after the distillation of lighter oils such as gasoline and diesel. It is commonly used as a fuel in power plants and industrial boilers. Alternative terms for residual oil include low-sulfur residual fuel oil, bunker fuel, heavy fuel oil, and boiler fuel. These terms are often used interchangeably, but bunker fuel specifically refers to the fuel used in ships' engines. Heavy fuel oil is also used to describe residual oil, but can also include other very thick, viscous petroleum products. Boiler fuel is a more general term used to describe any fuel burned to heat water and generate steam for use in industrial processes.
