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Consequently, aromatic steranes are not suitable for sample comparison in cases where pure 
lubricating oil is found in an oil spill. 
But the same applies to light fuel oil/lubricating oil mixtures, i.e. waste oil which is typically 
present in the bilges of smaller ships. Instead, the finding that aromatic steranes are not 
present in lubricating oil is particularly useful even in the recognition of waste oil (bilge oil, 
sludge, see Paragraph 2.4). 
2.3 Heavy fuel oil (HFO, Bunker C, Fuel No 6) 
Heavy fuel oils are blended products derived from the residues of various refinery distillation 
and cracking processes. The term Intermediate Fuel Oil (IFO) is often used in this respect. 
They are highly viscous liquids with a characteristic odour and black colour (black oil) which 
require heating for storage and combustion. Heavy fuel oils are used in medium to large 
industrial plants, marine applications and power stations in combustion equipment such as 
boilers, furnaces, and diesel engines. 
Heavy fuel oil consists primarily of residues from the distillation and cracking units of 
refineries. Historically, fuel oils have been based on residues from the atmospheric distillation 
column and were also called straight-run fuels. However, rising demand for transportation 
fuels such as gasoline, kerosine and diesel has increased the value of atmospheric residues as 
a feedstock in vacuum distillation and cracking processes. As a consequence, most heavy fuel 
oils are currently based on residues from vacuum distillation units and from thermal and 
catalytic cracking operations. 
Different grades of HFOs are expressed by numbers representing their kinematic viscosity in 
cSt at 50 °C. The main grades are IFO30, IFO180 and IFO380. Although there may be 
exceptions, the heavier-grade IFO380 is used as bunker oil by the vast majority of ships over 
5,000 tons gross tonnage. 
The use of heavy fuel oil as bunker oil on ships has been found to be the main cause of 
chronic oil pollution: 
Heavy fuel oil has to be cleaned of solid contaminants and water on board the vessel before 
being used (fuel oil separation by centrifuging). The solid contaminants are mainly residual 
sediments, refinery catalysts and high-boiling asphaltenes and resins. Thus, large amounts of 
residues and residual oil have to be stored on board until they can be discharged to harbour 
reception facilities. According to a worldwide study conducted by the International Maritime