Fuel oil (also known as heavy oil, marine fuel or furnace oil) is a
Although the following trends generally hold true, different organizations may have different numerical specifications for the six fuel grades. The boiling point and
Number 1 fuel oil is a volatile distillate oil intended for vaporizing pot-type burners.
 It is the
Number 2 fuel oil is a distillate home
Number 3 fuel oil was a distillate oil for burners requiring low-viscosity fuel. ASTM merged this grade into the number 2 specification, and the term has been rarely used since the mid-20th century. 
Number 5 fuel oil is a residual-type industrial heating oil requiring preheating to 77–104 °C (171–219 °F) 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. 
Number 6 fuel oil is a high-viscosity residual oil requiring preheating to 104–127 °C (219–261 °F). 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% water and 0.5% 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. 
The British Standard BS 2869, Fuel Oils for Agricultural, Domestic and Industrial Engines, specifies the following fuel oil classes:
|Class||Type||Min. kinematic viscosity||Max. kinematic viscosity||Min. flash point||Max. sulphur content||Alias|
|C1||Distillate||—||—||43 °C||0.040 % (m/m)||Paraffin|
|C2||Distillate||1.000 mm²/s at 40 °C||2.000 mm²/s at 40 °C||38 °C||0.100 % (m/m)||
|A2||Distillate||2.000 mm²/s at 40 °C||5.000 mm²/s at 40 °C||> 55 °C||0.001 % (m/m)||low-sulphur gas oil,
|D||Distillate||2.000 mm²/s at 40 °C||5.000 mm²/s at 40 °C||> 55 °C||0.100 % (m/m)||Gas oil,
|E||Residual||—||8.200 mm²/s at 100 °C||66 °C||1.000 % (m/m)||Light fuel oil, LFO, 250-second oil|
|F||Residual||8.201 mm²/s at 100 °C||20.000 mm²/s at 100 °C||66 °C||1.000 % (m/m)||Medium fuel oil, MFO, 1000-second oil|
|G||Residual||20.010 mm²/s at 100 °C||40.000 mm²/s at 100 °C||66 °C||1.000 % (m/m)||Heavy fuel oil, HFO, 3500-second oil|
|H||Residual||40.010 mm²/s at 100 °C||56.000 mm²/s at 100 °C||66 °C||1.000 % (m/m)||—|
Class C1 and C2 fuels are kerosene-type fuels. C1 is for use in flueless appliances (e.g.
Class A2 fuel is suitable for mobile,
Classes E to H are residual oils for atomizing burners serving boilers or, with the exception of Class H, certain types of larger combustion engines. Classes F to H invariably require heating prior to use; Class E fuel may require preheating, depending on ambient conditions.