The term fuel oil is frequently used in a stricter sense to refer to the heaviest commercial fuel that can be obtained from crude oil (i.e. heavier than gasoline and naphtha).
The ASTM classification system, originally based on early refining and combustion engineering practices and nomenclature, identifies 6 fuel oil specifications.

Price usually decreases as the fuel number increases.
  • NUMBER 1: a volatile distillate oil intended for vaporizing pot-type burners. It is the kerosene refinery cut that boils off right after the heavy naphtha cut used for gasoline. Older names include coal oil, stove oil and range oil.
  • NUMBER 2: a distillate home heating oil. Trucks and some cars use similar diesel fuel with a cetane number limit describing the ignition quality of the fuel. Both are typically obtained from the light gas oil cut.
  • NUMBER 3: a distillate oil for burners requiring low-viscosity fuel. ASTM merged this grade into the number 2 specification and the term is rarely used.
  • NUMBER 4: a commercial heating oil for burner installations not equipped with preheaters. It may be obtained from the heavy gas oil cut.
  • NUMBER 5: a residual-type industrial heating oil requiring preheating to 170 – 220 °F (77 – 104 °C) for proper atomization at the burners. This fuel is sometimes known as Bunker B. It may be obtained from the heavy gas oil cut, or it may be a blend of residual oil with enough number 2 oil to adjust viscosity until it can be pumped without preheating. Also called Navy Special Fuel Oil, Navy Special or Furnace Fuel Oil.
  • NUMBER 6: a high-viscosity residual oil requiring preheating to 220 – 260 °F (104 – 127 °C). Residual means the material remaining after the more valuable cuts of crude oil have boiled off. The residue may contain various undesirable impurities including 2 percent water and one-half percent mineral soil. This fuel may be known as residual fuel oil (RFO), by the Navy specification of Bunker C, or by the Pacific Specification of PS-400. Also called Furnace Fuel Oil.


Regularly referred to as No. 6 fuel oil, which is the most common bunker fuel. These are also a type of fuel oil, used aboard ships.
  • Bunker A: No. 2 fuel oil.
  • Bunker B: No. 4 or 5 fuel oil.
  • Bunker C: No. 6 fuel oil.


Other types of classification used in the maritime field also include:
  • HFO: Heavy Fuel Oil - pure or nearly pure residual oil, roughly equivalent to No. 6 fuel oil.
  • IFO: Intermediate Fuel Oil - a blend of gasoil and heavy fuel oil, with less gasoil than marine diesel oil.
  • MDO: Marine Diesel Oil - a blend of heavy gasoil that may contain very small amounts of black refinery feed stocks, but has a low viscosity up to 12 cSt so it need not be heated for use in internal combustion engines.
  • MFO: Marine Fuel Oil - same as HFO.
  • MGO: Marine Gas Oil - roughly equivalent to No. 2 fuel oil, made from distillate only.
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