Used oil analysis is comparable to a medical analysis with a blood test. Like blood, lubricating oil contains a good deal of information about the envelope in which it circulates. Wear of metallic parts, for example, produces a lot of minute particles, which are carried by the lubricant. These small metal particles can give information about the machine elements that are wearing, and can be detected by various methods, for example, Atomic Emission Spectrometry. Determination of larger particles can be done using optical or electronic microscopy, or ferrography

The acidity of an oil shows whether the oil is oxidised as a result of operation at high temperature, if there is a high percentage of moisture, or whether the oil has been in service for too long. The viscosity of the oil is a very important parameter and must be in conformity with the requirements of the machine builder. The alkalinity or the loss of alkalinity of the oil, proves that the oil is in contact with inorganic acids such as sulphuric acid or nitric acid.

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

Oils undergo destructive changes in property when subjected to oxygen, combustion gasses and high temperatures. Viscosity change, as well as additive depletion and oxidation occur to degrade the oil.