15 
used to reduce the wax content of the base oils so as to prevent wax crystals from forming 
within the normal working temperature range of the lubricant. 
Higher viscosity grades of lubricating oils are produced by solvent deasphalting of the 
vacuum residue of crude oil with liquid propane. The liquid propane precipitates the resins 
and asphaltenes to produce a deasphalted residual oil. This is then further treated by solvent 
extraction and/or hydrogenation to reduce the aromatics content. Residual oils are commonly 
used in the heavier grades of lubricant, for example, marine diesel engine oils and heavy gear 
oils. 
In this paragraph, no differentiation is made between lubricating oil and hydraulic oil because 
these oils contain the same base oils. 
Pure, unused lubricating oil has been found only in very rare cases of oil pollution because 
lubricating oil is a valuable, expensive product. On the other hand, it is normal practice on 
board ships to first collect wastes from all kinds of oil in collecting tanks (sludge tanks, bilge 
oil tanks). Thus, lubricating oil is often found as part of pollution by waste oil. 
The following description, therefore, focuses mainly on possibilities of identifying lubricating 
oil in waste oil. The presence of lubricating oil together with bunker oil in an oil spill sample 
is strong evidence of waste oil having been discharged. 
2.2.1 Analysis 
2.2.1.1 GC screening 
Like light fuel oil, lubricating oil is easily detected in GC screening because of its limited 
boiling range. But in this case, the higher boiling region is covered. N-alkanes are only 
present in traces, and the whole oil forms a relative narrow UCM in the chromatogram. The 
boiling range covered is determined by the viscosity of the oil. Fig. 8 shows gas 
chromatograms of base oils (“base stocks”) with a kinematic viscosity of 90, 130, 150 and 
600, respectively (cSt at 40 °C, upper to lower and left to right).