Marine fuels

Residual oil: it is the heaviest fraction of the distillation of crude oil, with high viscosity (=> pre-heating necessary => used only in large ships) and high concentration of pollutants (e.g. sulphur). Its combustion produces a much darker smoke than other fuels and it needs specific temperature for storage and pumping. Due to these drawbacks, it is also the cheapest liquid fuel on the market.
IFO 180 (Intermediate Fuel Oil) : it is a mix of 98% of residual oil and 2% of distillate oil.
IFO 380 (Intermediate Fuel Oil) : it is a mix of 88% of residual oil and 12% of distillate oil. Due to the higher content in distillate oil, IFO 380 is more expensive than IFO 180.
MDO (Marine Diesel Oil) : it mainly consists of distillate oil and has a lower sulphur content than the three fuels described above.
MGO (Marine Gas Oil) : it is pure distillate oil and has the lowest sulphur content.

(ICCT, Air Pollution and GHG Emissions from Ocean-going Ships: Impacts, Mitigation Options and Opportunities for Managing Growth, March 2007)