The basic properties and main selection principle of industrial lubricating oil are viscosity. Therefore, priority must be given to introduce it. GB/T3141-94 is the ISO viscosity classification of industrial liquid lubricants, equivalent to the international standard ISO3448-1992 "Industrial liquid lubricant-ISO viscosity classification." Generally speaking, under the condition of medium speed, medium load and low temperature, medium viscosity lubricant is selected; Under the condition of high load, low speed and high temperature, high viscosity lubricating oil or lubricating oil with high pressure abrasives are selected. Under the condition of low load, high speed and low temperature, low viscosity lubricant is chosen. Synthetic lubricating oil is selected for wide, high, low temperature range, light load, high speed, and other special requirements.