The causes of oil contamination are many, and can be classified according to source. Thus there is contamination coming from outside the system - dust (silica); liquids (mixture with other oils, water, other contaminated oil). The second is in open systems - chains, cables, gears in contact with dust, water, and so on. The third is in closed systems.
Impurities can also come from the settings and processes in which the lubricants work, e.g. manufacturing can produce welding debris, assembling involves dust, perhaps also silicones or polishing powder, while maintenance can introduce impurities via dirty rags or deteriorated joints, and lubrication systems may need or involve aspiration, or open tanks.
The lubricant itself can produce or contain contaminants - wear, sludge (deterioration of the oil), soot, acids (oxidation of the oil, sulphur from fuel), temperature changes or extremes, fuel, anti-freeze, deterioration of packings and seals (e.g. deteriorating through the action of synthetic oils or brake fluids).
The type of contamination can vary according to the source. Thus dust, for example, can arise within a shipyard as sand, i.e. Si, Al. In metallurgy, one can find the oxides or iron. On a ship, there are problems with salt water. Industrial and automotive settings are filled with potential contaminants, for example chemical products, or coal powder. Liquid contaminants can include water, acids, solvents, anti-freeze.