fuel oil

Residual refinery product burned for the generation of heat, and having a flash point above 40°C (104°F). Residential use fuel oils are classified as Number 1, 2, or 3, and industrial ones are classified as Number 4, 5, or 6 according to their specific gravity (higher the number, higher the sp. gravity). Number 5 and 6 are used mainly for powering ships and are also called bunker oils.