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Typically, equipment manufacturers specify the lubricating oils for various parts of the equipment in their manuals, and equipment lubrication managers only need to select the appropriate oils as required by those manuals. This is often due to: 1) incomplete manuals for certain equipment or no specification regarding the oil to be used. 2) After the oil specified in the manual is used up. 3) For various reasons, it is not possible to use the oil as specified in the instructions (for example, foreign oils recommended for imported equipment that are difficult to obtain domestically, or domestic oils that must be used to achieve localization). At this time, oil can be selected based on the following principles. 1. Select according to the operating conditions of the equipment: (1) Load – For high loads, choose oils with high viscosity and good oiliness or extreme pressure properties ； If the load is low, choose oil with a lower viscosity ； In applications with high stress, oils with high viscosity and excellent extreme pressure properties should also be chosen. (2) Operating speed: For high speeds, choose an oil with low viscosity; for components operating at low speeds, an oil with higher viscosity can be used. However, for oils containing anti-wear additives, there is no need to emphasize a high viscosity excessively. (3) Temperature: Temperature is divided into ambient temperature and operating temperature. When the ambient temperature is low, choose lubricants with lower viscosity and freezing point (or pour point); otherwise, a higher value is acceptable ； At high operating temperatures, lubricants with higher viscosity, higher flash points, and better oxidation stability should be chosen; solid lubricants can even be used ； For applications with large temperature variations, lubricants with good viscosity-temperature properties (high viscosity index) should be selected. (4) Environmental humidity and water contact: In humid environments or under conditions involving frequent contact with water, lubricants with strong anti-emulsification properties, as well as good oiliness and rust prevention capabilities, should be selected. 2. Choose the oil as recommended in the equipment manual. The oil type suggested in the equipment manual can serve as a main guide for making this choice, but it should be noted that with technological advancements, inferior oils will gradually be phased out; therefore, it is economical to choose high-quality oils. Therefore, even with old equipment, obsolete and low-quality oils should not be continued to be used ； Lubricants for imported and advanced equipment should prioritize domestic products. 3. Select the type of lubricant and its viscosity grade based on the application. Domestic lubricants are named using coding symbols according to their application, composition, and properties. Therefore, when making a selection, one can first determine the group based on the application scenario, and then select the type and viscosity grade according to the operating conditions. In lubricant management, once the appropriate oil type has been selected, it is generally advisable to avoid using substitutes or mixing different oils. But sometimes it is necessary to use alternative or mixed oils due to supply issues or other reasons; in such cases, the following principles should be followed: 1) Only oils of the same type, or those with similar properties and similar types of additives, can be used as substitutes or mixed together. 2) The viscosity of the substitute oil should not exceed 25% above that of the original oil; generally, a substitute oil with slightly higher viscosity can be used, but for hydraulic oil and spindle oil, it is preferable to choose a substitute oil with slightly lower viscosity. 3) In terms of quality, only higher standards can replace lower ones; lower standards cannot replace higher ones. For machinery with large variations in operating temperature, only those with good viscosity-temperature properties can be used instead of those with poor such properties ； For use in low-temperature environments, alternative oils should have a freezing point or pour point that is 10 degrees lower than the operating temperature℃ ； For high-temperature operation, substitute oils with a high flash point and good oxidation stability and thermal stability should be selected. 4) Since lubricants with the same name produced by different manufacturers may contain different additives, it is advisable to conduct a mixing test before mixing new oil from various manufacturers with old oil. This involves mixing them in a 1:1 ratio, heating and stirring the mixture, and checking for any abnormalities such as unusual odors or sediment; only if no such issues are present can the oils be used together.