From wikipedia.

In the maritime field another type of classification is used for fuel oils:

* MGO (Marine gasoil) - roughly equivalent to No. 2 fuel oil, made from distillate only
* MDO (Marine diesel oil) - A blend of gasoil and heavy fuel oil
* IFO (Intermediate fuel oil) A blend of gasoil and heavy fuel oil, with less gasoil than marine diesel oil
* MFO (Medium fuel oil) - A blend of gasoil and heavy fuel oil, with less gasoil than intermediate fuel oil
* HFO (Heavy fuel oil) - Pure or nearly pure residual oil, roughly equivalent to No. 6 fuel oil

Marine diesel oil contains some heavy fuel oil, unlike regular diesels. Also, marine fuel oils sometimes contain waste products such as used motor oil.

[edit] Standards and classification

Marine fuels were traditionally classified after their kinematic viscosity. This is a mostly valid criteria for the quality of the oil as long as the oil is made only from atmospheric distillation. Today, almost all marine fuels are based on fractions from other more advanced refinery processes and the viscosity itself says little about the quality as fuel. CCAI and CII are two indices which describe the ignition quality of residual fuel oil, and CCAI is especially often calculated for marine fuels. Despite this marine fuels are still quoted on the international bunker markets with their viscosity due to the fact that marine engines are designed to use different viscosities of fuel. [1]. The unit of viscosity used is the Centistoke and the fuels most frequently quoted are listed below in order of cost, the least expensive first-

* IFO 180 - Intermediate fuel oil with a viscosity of 180 Centistokes
* LS 180 - Low Sulphur intermediate fuel oil with a viscosity of 180 Centistokes
* IFO 380 - Intermediate fuel oil with a viscosity of 380 Centistokes
* LS 380 - Low Sulphur intermediate fuel oil with a viscosity of 380 Centistokes
* MDO - Marine diesel oil.
* MGO - Marine gasoil.

The density is also an important parameter for fuel oils since marine fuels are purified before use to remove water and dirt from the oil. Since the purifiers use centrifugal force, the oil must have a density which is sufficiently different from water. Older purifiers had a maximum of 991 kg/m3; with modern purifiers it is also possible to purify oil with a density of 1010 kg/m3.

The first British standard for fuel oil came in 1982. The latest standard is ISO 8217 from 2005. The ISO standard describe four qualities of distillate fuels and 10 qualities of residual fuels. Over the years the standards have become stricter on environmentally important parameters such as sulfur content. The latest standard also banned the adding of used lubricating oil (ULO).

Some parameters of marine fuel oils according to ISO 8217 (3. ed 2005):
Marine Distillate Fuels
Parameter Unit Limit DMX DMA DMB DMC
Density at 15°C kg/m3 Max - 890.0 900.0 920.0
Viscosity at 40°C mm²/s Max 5.5 6.0 11.0 14.0
Viscosity at 40°C mm²/s Min 1.4 1.5 - -
Water % V/V Max - - 0.3 0.3
Sulfur1 % (m/m) Max 1.0 1.5 2.0 2.0
Aluminium + Silicon2 mg/kg Max - - - 25
Flash point3 °C Min 43 60 60 60
Pour point, Summer °C Max - 0 6 6
Pour point, Winter °C Max - -6 0 0
Cloud point °C Max -16 - - -
Calculated Cetane Index Min 45 40 35 -

1. Max sulfur content is 1.5% in designated areas.
2. The aluminium+silicon value is used to check for remains of the catalyst after catalytic cracking. Most catalysts contains aluminium or silicon and remains of catalyst can cause damage to the engine.
3. The flash point of all fuels used in the engine room should be at least 60°C (DMX is used for things like emergency generators and not normally used in the engine room).
Marine Residual Fuels
Parameter Unit Limit RMA 30 RMB 30 RMD 80 RME 180 RMF 180 RMG 380 RMH 380 RMK 380 RMH 700 RMK 700
Density at 15°C kg/m3 Max 960.0 975.0 980.0 991.0 991.0 991.0 991.0 1010.0 991.0 1010.0
Viscosity at 50°C mm²/s Max 30.0 30.0 80.0 180.0 180.0 380.0 380.0 380.0 700.0 700.0
Water % V/V Max 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
Sulfur1 % (m/m) Max 3.5 3.5 4.0 4.5 4.5 4.5 4.5 4.5 4.5 4.5
Aluminium + Silicon2 mg/kg Max 80 80 80 80 80 80 80 80 80 80
Flash point3 °C Min 60 60 60 60 60 60 60 60 60 60
Pour point, Summer °C Max 6 24 30 30 30 30 30 30 30 30
Pour point, Winter °C Max 0 24 30 30 30 30 30 30 30 30

1. Max sulfur content is 1.5% in designated areas.
2. The aluminium+silicon value is used to check for remains of the catalyst after catalytic cracking. Most catalysts contains aluminium or silicon and remains of catalyst can cause damage to the engine.
3. The flash point of all fuels used in the engine room should be at least 60°C.