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.