Lubricating oil contamination is a complex issue, some of which can be detected during daily inspections, while others cannot be observed by the naked eye. Moreover, for all pollution, when it can be detected by the naked eye, it indicates that it is already very serious. In short, the pollution of lubricating oil should be detected and treated early, especially for sensitive and critical equipment.

Particulate matter is the most harmful pollutant, as it enters the lubrication system and can cause abrasive wear, metal pressure scratches, and metal fatigue. Many particles, although small in size, have a certain hardness and can pass through the gaps between parts, circulating inside the equipment and causing wear. Common particulate matter includes dust, gravel, small metal particles generated during equipment operation, rust residue, etc. These particulate pollutants not only harm the equipment itself, but also shorten the service life of lubricating oil.

（Left: Metal fatigue Right: Crater formed by metal surface wear）

Moisture is a common pollutant, although not as harmful as particulate matter, it can damage lubrication, deteriorate oil, cause equipment wear, and also cause metal corrosion. Lubrication relies on the formation of an oil film between metal contact surfaces, which separates direct friction between metals and prevents direct contact between them. If moisture enters the metal contact stress area of the bearing, it will damage the integrity of the oil film, reduce the strength of the oil film, lead to poor lubrication or direct friction between metals, resulting in metal scratching and fragmentation. In addition, water can cause the loss and consumption of antioxidants in lubricating oil, leading to the oxidation and deterioration of lubricating oil. Oxidation of lubricating oil can form acidic substances, sludge, and paint film, increase the viscosity of the oil, and affect the effectiveness of splash lubrication, etc.

When using lubricating oil, contact with other oils should be avoided. However, in some cases, such as leaks, using the wrong lubricant during refueling (incorrect selection of lubricant viscosity or wrong selection of additive type), etc., different lubricants can be mixed. For example, mineral oil is incompatible with conventional PAG synthetic oil (non oil soluble PAG). If these two types of oil are mixed, it will cause an increase in viscosity of the mixed oil and the formation of sludge. Other phenomena include an increase in acid value and blockage of the filter element by sludge. Meanwhile, due to poor lubrication caused by mixing, equipment wear may also occur.