How long will the engine oil "spoil" and is it reasonable to replace it according to the time interval

The engine oil of a car generally does not deteriorate unless there is water in the crankcase of the engine! Water will react with engine oil, leading to oil emulsification. Of course, there will be an increase in the total amount before emulsification, which is an increase in engine oil. On the contrary, without the occurrence of oil increase or emulsification in the engine, there is no concept of deterioration in the oil - whether it is oil or not, the oil will only experience performance degradation.

What kind of oil is engine oil?

Obviously, engine oil does not belong to vegetable oil or animal oil. Engine oil is a product of petroleum fractionation; That's why engine oil can burn. Many German cars use "engine oil+gasoline" to achieve "hybrid power". If engine oil cannot burn, then these engines will be scrapped long ago. Engine oil is mainly made from heavy oil, and all fuel oils belong to heavy oil, while naphtha belongs to light oil.

After treatment, heavy oil can be said to generate lubricating oil, tar, petroleum coke, gasoline, liquefied petroleum gas, etc. Now you know what lubricating oil is.

Petroleum is a mixture that can evaporate in room temperature air, but only light components such as naphtha and diesel can evaporate. Wax oil also evaporates or oxidizes, leaving behind asphalt; That is to say, in the natural environment, heavy oil is extracted, and asphalt is the residue after heavy oil extraction. Does this mean that lubricating oil can also be oxidized or evaporated? The previous discussion was only theoretical, as heavy oils with large molecules are not easily volatile; Heavy oil doesn't have to worry about oxygen, even if it's petroleum. To fully oxidize one liter of petroleum, you need over a thousand liters of oxygen, which is even more outrageous for lubricating oil.

And the oxygen concentration in atmospheric air is only less than 21%. Think about how many liters of air are needed to cause problems with lubricating oil.

However, some people say that the engine has a blowby design, and every time work is done, there will be a mixture of incompletely burned gases entering the crankcase and coming into contact with the engine oil; But combustion itself is an oxidation reaction, which has already consumed a large amount of oxygen. How much more oxygen can enter the crankcase?

Lubricating oil is not only base oil, but it is generally understood as base oil. There is no need to worry too much about the role of oxygen in this part. In fact, the role of light is much greater than that of oxygen, but the oil in the crankcase cannot see light; The other part of engine oil is various additives, such as antioxidants! This is enough to explain the problem. Secondly, there are extreme pressure anti wear agents, emulsifiers, defoamers, dispersants, and so on.

This type of lubricating oil does not have to worry about "spoilage" due to oxygen, and the engine oil is still engine oil at all times.

If you don't believe it, get to know the gearbox. Transmission oil is also a lubricating oil, collectively known as engine oil; The transmission oil of a typical AT can reach 80000 to 100000 kilometers, but many household cars cannot run 10000 kilometers a year, so many household cars have not changed the transmission oil. But the gearbox passes through a pressure relief port, and external air or even water can enter the gearbox. Of course, as long as driving without wading in water, there is generally no water ingress.

Don't you need to worry about "deterioration after a long time" for transmission oil?

Suggested standards:

The recommended usage period for engine oil is about two years, and in extreme cases it can be used for more than three years.

The mileage should be based on standards, with mineral oil suitable for 5000 kilometers, semi synthetic oil suitable for general small and medium-sized naturally aspirated engines for 10000 kilometers, and turbocharged engines for 5000 kilometers; Fully synthetic engine oil can be used for any engine for more than 10000 kilometers. In fact, many old vehicles equipped with naturally aspirated engines can use 20000 kilometers without any problem, but it is generally not recommended to use such a long mileage.