Under long-term operation, air will seep into the engine oil, oxidizes engine oil and causes oil mud. The piston is pressed down under high temperature and high pressure after the engine's explosion, at this moment, the oil film is easily carbonized due to high-temp and high pressure and adhere to the surface of engine parts, which leads to oil mud.
Analyze the components of the oil mud, you will discover the fine sand, which is flying dust got in from intake valve; there are some metal substances, those might be the scraps coming from the engine parts friction, or the process of making engines without cleaning up completely.
The hard object is bad for the engine. The main component of engine oil is the long chain of hydrocarbons. If the length of chains varies, the strength is different. Once the chains get collided, the weak one will break, and take away the other chain's position. This will lead molecular reorganization and produce hydrocarbons with branches. If the branches are too many, they will get together due to the polar attraction, thus oil mud is produced.
At room temperature, the process goes slowly, and the more uniform the oil components, the less likely to occur. For the engine, high temperature is the main cause of broken chains and reorganization. Generally speaking, the engine oil is actually resistant to high temperature. But when the cylinder exudes gas, there's many thermal decomposition of hydrocarbon bonds. These substances meet the requirements of reorganization with engine oil molecular and recombine into branched structure molecules.
In addition, engine oil molecular might be partly oxidized or get acid Ionization. This will completely change the molecular structure and produce stronger attraction between molecules. This is not an inevitable mechanism for the production of sludge, but it should be the most awful mechanism for producing sludge.
