fuel oil - term encompassing a broad range of distillate and residual fuels identified by ASTM grades J through 6. Grade No. 1, a kerosene-type fuel, is a light distillate fuel that has the lowest boiling range. No. 2 fuel oil, popularly called heating oil, has a higher boiling range and is commonly used in home heating. It is comparable in boiling range to diesel fuel. Grades 4, 5, and 6 are called heavy fuel oils (HFO), or residual fuel oils; they are composed largely of heavy pipe still bottoms. Because of their high viscosity, No. 5 and No. 6 fuel oils require preheating to facilitate pumping and burning. No. 6 fuel oil is also called Bunker C fuel oil. See distillation.