Fuel oil is any liquid petroleum fuel burned in a furnace, boiler or engine to produce heat or power, from light heating oil to the heavy black oil used by ships and power stations. In the trade the term most often means the heavy end: residual fuel oil, the material left after a refinery has distilled off petrol, kerosene and diesel. In the United States, fuel oils are numbered from No. 1, the lightest, to No. 6, the heaviest.
What is fuel oil made from?
Fuel oil comes from the distillation of crude oil, and it falls into two families.
- Distillate fuel oils have been boiled off and condensed in the refinery. They are thin, clean and light in colour. Kerosene, heating oil, gasoil and marine gasoil are distillates.
- Residual fuel oils are what remains at the bottom of the distillation columns. They are black, viscous and carry most of the crude oil's sulphur (US: sulfur) and metals. They usually have to be heated before they can be pumped or burned.
Many commercial grades are blends of the two. A refiner or blender adds a lighter distillate, called cutter stock, to a heavy residue to bring the viscosity down to the grade required.
"Fueloil", written as one word, is the same product. The one-word spelling appears in some trade documents and offers.
The numbered grades: No. 1 to No. 6
ASTM D396 is the US standard specification for fuel oils used in burners. Its scope describes the grades in three groups: No. 1 and No. 2 as middle distillates for domestic and small industrial burners; No. 4 as heavy distillate or a blend of distillate and residual fuel; and No. 5 and No. 6 as residual fuels of increasing viscosity, for which preheating is usually required.
| Grade | Type | Common names | Notes |
|---|---|---|---|
| No. 1 | Distillate | Kerosene-type heating oil | Lightest grade; for vaporising burners and low-temperature storage |
| No. 2 | Distillate | Home heating oil, gasoil | Close to road diesel in boiling range |
| No. 3 | Distillate | None in current use | Merged into No. 2; the term is rarely used now |
| No. 4 | Heavy distillate or blend | Bunker A | Commercial heating oil for burners without preheaters |
| No. 5 | Residual | Bunker B, Navy Special | Medium viscosity; preheated to 77 to 104°C |
| No. 6 | Residual | Bunker C, residual fuel oil | Heaviest grade; preheated to 104 to 127°C |
What is No 6 fuel oil?
No. 6 fuel oil is the heaviest of the numbered grades, a high-viscosity residual oil. It is also known as Bunker C. The US Energy Information Administration lists its uses as electric power generation, space heating, vessel bunkering and various industrial purposes.
No. 6 will not flow freely at ambient temperature. It is typically stored at around 38°C and heated to between 65 and 120°C before pumping, and it has a flash point of about 65°C. Its specific gravity is usually between 0.95 and 1.03, so some cargoes are denser than water.
What is number 5 fuel oil?
Number 5 fuel oil is a residual fuel of medium viscosity, lighter than No. 6. It is also called Bunker B or Navy Special, and ASTM D396 divides it into a light and a heavy grade. The US Energy Information Administration gives its uses as steam-powered vessels in government service and inshore power plants. Marine and export buyers do not usually order by this number; they specify a viscosity and a sulphur content instead.
Marine fuel oil grades under ISO 8217
Ships do not buy by ASTM number. Marine fuels are specified under ISO 8217, the international marine fuel standard, of which ISO 8217:2024 is the latest edition. It divides fuels into distillate and residual categories.
The trading names combine viscosity and sulphur:
| Trade name | What it is | Viscosity | Sulphur |
|---|---|---|---|
| HSFO 380 (IFO 380) | Residual fuel blended to grade | 380 cSt maximum at 50°C | Up to 3.5%; only for ships with scrubbers |
| VLSFO | Very low sulphur fuel oil, residual or blended | Varies by blend | 0.50% maximum |
| MGO | Marine gasoil, distillate only | Low; pumps without heating | Low sulphur grades of 0.10% for emission control areas |
IFO stands for intermediate fuel oil, and the number is the maximum viscosity. ISO 8217 allows residual grades up to 700 cSt at 50°C and a maximum density of 1010 kg/m³ at 15°C, with a minimum flash point of 60°C.
Our product pages give the specification lines for each: IFO 380 bunker fuel, HSFO 3.5%, VLSFO 0.5% and marine gasoil.
The IMO 2020 sulphur cap
Sulphur in fuel becomes sulphur oxides in the exhaust. The International Maritime Organization (IMO) limits it under Regulation 14 of MARPOL Annex VI.
- Global limit. From 1 January 2020 the sulphur content of fuel oil used on ships outside emission control areas may not exceed 0.50% by mass. The previous limit was 3.50%.
- Emission control areas (ECAs). Inside an ECA the limit is 0.10%. The ECAs are the Baltic Sea, the North Sea, the North American area and the US Caribbean Sea area. The Mediterranean Sea became a sulphur ECA with the limit applying from 1 May 2025.
- Carriage ban. From 1 March 2020 a ship may not carry non-compliant fuel for use on board unless it has an exhaust gas cleaning system.
- Scrubbers. An exhaust gas cleaning system, or scrubber, is accepted as an equivalent method. A ship fitted with one may continue to burn high sulphur fuel oil.
This rule split the bunker market into the three products in the table above. HSFO is now a fuel for scrubber-fitted ships, VLSFO is the standard fuel for the rest of the fleet, and MGO is used in ECAs and in engines that need a distillate.
What does cSt mean?
cSt is the centistoke, the unit of kinematic viscosity, which measures how easily a liquid flows under its own weight. One centistoke equals one square millimetre per second (mm²/s). The higher the figure, the thicker the oil.
Viscosity falls quickly as oil is heated, so a figure means nothing without its temperature. Residual marine fuels are quoted at 50°C. A buyer comparing two offers should check that both state viscosity at the same temperature.
What is fuel oil used for?
- Shipping. Bunker fuel for the main engines and boilers of ocean-going vessels.
- Power generation. Oil-fired power stations and standby generation.
- Industry. Boilers, kilns and furnaces in industrial plants.
- Heating. The distillate grades, No. 1 and No. 2, for domestic and commercial heating.
- Refinery feedstock. Straight-run residues are bought by refiners as feed for cracking and coking units.
What a fuel oil specification covers
Whatever the grade, a specification sheet for fuel oil carries the same core lines: viscosity at a stated temperature, density at 15°C, sulphur, flash point, pour point, water, ash and metals such as vanadium. For marine fuels the stability and compatibility of the blend also matter, because two fuels that are each on specification can form sludge when mixed in a ship's tanks.
How fuel oil is traded
Fuel oil is sold per tonne, in cargo lots by tanker on FOB, CFR or CIF terms, or delivered to a ship as bunkers at a port. The contract names the grade, the standard and edition, and the limits that apply.
CBIL is a commodity brokerage company. We broker fuel oil between a named refinery or seller and a named buyer, against independent inspection by SGS, Intertek or a similar company at loading. The inspector gauges the shore tanks and the vessel, draws samples and tests them against the contract specification. The grades are listed on our fuel oil page.
Request a fuel oil quote with the grade, sulphur limit, viscosity, quantity and delivery port.