Heavy Fuel Oil (HFO)

Heavy fuel oil (HFO), or “residual fuel oil” is based on the high viscosity, tar-like mass which remains after crude oil has been cracked to produce lighter hydrocarbon products such as methane, hydrogen, petrol (gasoline), distillate diesel fuels and heating oils or feedstocks for lubricants.

Source: http://www.mandieselturbo.com/0000602/Products/Power-Plant-Engines/Fuels/HFO.html