The viscosity index indicates the degree to which the viscosity of all fluids changes with temperature. The higher the viscosity index is, the less the fluid viscosity is affected by temperature, and the less sensitive the viscosity is to temperature.
According to different viscosity index, lubricating oil can be divided into three grades: 35-80 is medium viscosity index lubricating oil; 80-110 refers to lubricating oil with high viscosity index; Above 110 is a super high viscosity index lubricating oil. The engine oil with viscosity index of 100-170 is a high-grade multistage lubricating oil. It has smooth change of viscosity temperature curve and good viscosity temperature property. At lower temperature, the solubility of macromolecular organic compound molecules in the oil of these viscosity index improvers is small, and the molecules curl up into compact small groups, so the viscosity of the oil increases little; At high temperature, its solubility in the oil increases, and the curly linear molecules expand and elongate, thus increasing the viscosity greatly, making up for the reduced viscosity of the base oil due to temperature rise. Therefore, the higher the viscosity index, the smaller the viscosity changes with temperature.