Oxidation is the main cause of deterioration of lubricating oil, and the lubricating oil is oxidized due to exposure to air during use. For mineral lubricating oils, oxidation can produce macromolecular incompetence, which usually increases the viscosity of lubricating oil. For some synthetic oils, such as polyalphaolefins, polyester synthetic oils, etc., high temperature will cause long-chain molecular cracking of base oils. It becomes a structure of small molecules, so in the initial stage of oxidation, the viscosity will decrease.

Viscosity is the most important indicator of lubricating oil. The viscosity of lubricating oil should be kept within the proper range to achieve good lubrication. Under normal circumstances, the oxidation of lubricating oil is a very slow process, and the effect on viscosity is not Very obvious. However, high temperature, moisture, particulate contaminants, etc. will accelerate the oxidation and deterioration of the lubricating oil. Therefore, when using lubricating oil, care should be taken to avoid excessive oil temperature, keep the lubricating oil clean and free of water. The lubricating oil after opening should be used up as soon as possible. If it is not used, it should be sealed and stored in a cool, clean and dry place to avoid sunlight and prevent the lubricating oil from oxidizing too quickly and deteriorate.

If the viscosity of the lubricating oil has changed significantly (more than 10% of the change), it must be taken seriously. It is necessary to judge whether it is a new oil or a full oil change according to the degree of oxidation.