Residual Fuel Oil Definition:A general classification for the
heavier oils, known as No. 5 and No. 6 fuel oils, that remain
after the distillate fuel oils and lighter hydrocarbons are
distilled away in refinery operations. It conforms to ASTM
Specifications D 396 and D 975and Federal Specification
VV-F-815C. No. 5, a residual fuel oil of medium viscosity, is
also known as Navy Special and is defined in Military
Specification MIL-F-859E, including Amendment 2 (NATO
SymbolF-770). It is used in steam-powered vessels in government
service and inshore powerplants. No. 6 fuel oil includes Bunker
C fuel oil and issued for the production of electric power,
space heating, vessel bunkering, and various industrial
purposes.