High temperature effect
①The lubricating oil will be oxidized and fail in a high temperature environment for a long time, and the phenomenon of blackening and thickening will appear.
② Corrosive substances inside the lubricating oil increase, such as acidic substances formed during the operation of the engine.
③The increase of carbon deposits, sludge, paint film and other substances.

impurities
Mainly from dust in the air, metal abrasive particles, seepage (fuel oil, water, etc.), lubricant oxides and substances produced by fuel combustion.
Additive failure
Some lubricating oils lose performance because the additives in them fail or run out. For example, when the anti-wear agent in the lubricating oil is used up, the anti-wear property will decrease.

Viscosity Index Improver Failure
Because the long chain of its organic molecules is broken, it no longer has the effect of increasing viscosity.
base oil failure
The base oil is the carrier of the additive, and the additive will not work if the base oil fails.
Although there are national standards to determine whether to replace the engine's lubricating oil, most car owners or maintenance personnel mainly judge the oil change based on the mileage/time of the car (specified by the manufacturer) and the color and viscosity of the lubricating oil. In fact, the above methods are all unreasonable. For example, according to the color judgment, some brand new lubricating oils appear black, and some lubricating oils may turn black after adding too much detergent and dispersant, but the lubricating oil does not fail at this time, which may cause misjudgment for inexperienced people. A simpler and more effective way is to use filter paper or electronic instruments to judge whether the lubricating oil fails.