When talking about the viscosity of lubricating oil, it generally involves the following three main concepts:

One is the viscosity of the lubricant itself, which refers to the measure of the resistance of the lubricant to flow (also called internal friction). In general, viscosity decreases with increasing temperature. The second is dynamic viscosity, which represents the measure of internal friction when the lubricant flows under a certain shear stress. It is calculated as the ratio of the applied shear stress to the shear rate of the flowing liquid, and its unit is mPa·s. The third concept, most widely used in practice, is kinematic viscosity. It represents the measure of internal friction of liquid molecules when the lubricant flows under the influence of gravity. Its value is the ratio of the dynamic viscosity of the liquid to its density at the same temperature, usually expressed in mm²/s in the International System of Units. The main factors affecting viscosity are as follows: temperature, pressure, and shear rate.

When talking about the viscosity of lubricating oil, we must also mention another very important concept that seems similar to viscosity but is completely different, namely the Viscosity Index (VI) of lubricating oil. The viscosity index characterizes how much the viscosity of the oil changes with temperature and can largely measure the oil's viscosity-temperature performance. However, generally speaking, only at the same temperature and when the viscosities are roughly similar can the viscosity index distinguish the viscosity-temperature performance. In the actual process of formulating oils, when one or several oils cannot meet the viscosity requirements, an appropriate amount of viscosity index improvers will be added for adjustment. For example, the highest viscosity index of a type of base oil is around 100, which generally can only be made into single-grade oil. After adding viscosity index improvers, the viscosity index can be increased to improve viscosity-temperature performance, allowing it to be formulated into multi-grade oils, suitable for use in all seasons. The main molecular structures of viscosity index improvers in the market are as follows: Olefin Copolymer (OCP), Poly(methyl acrylate) (PMA), and Hydrogenated Styrene-Isoprene Copolymer (HSD).

The tackifiers of the three configurations differ in terms of thickening ability, shear stability, and low-temperature performance, each with its own characteristics. The OCP configuration products have outstanding thickening ability and are more suitable for formulating various diesel engine oils and lubricants. PMA configuration products have certain advantages in low-temperature performance and are more commonly used in industrial lubricants, such as low-temperature hydraulic oils, gear oils, and transmission oils. In terms of shear stability, which is a very important property for lubricants, HSD configuration tackifiers perform better and are suitable for formulating high-end passenger car oils. (Due to the limited level of the author, there may be errors in the above content; please kindly point them out.)