Viscosity refers to the degree of "thickness" or "stickiness" of a fluid. A lubricant with high viscosity is thicker and more viscous than one with lower viscosity. There are two types of viscosity: dynamic and kinematic viscosity. Dynamic viscosity is measured in millipascal-seconds (mPa·s) or centipoise (cP), while kinematic viscosity is expressed in square millimetres per second (mm²/s) or centistokes (cSt).

Choosing the right lubricant, with the appropriate viscosity and viscosity index, depends on various factors. For new applications or troubleshooting, it is advisable to seek expert advice, considering all aspects, including load patterns, environmental factors, and safety requirements.

For lubricants, viscosity, together with the viscosity index, is a crucial parameter. A lubricant must not become too thin under normal operating temperatures, as it would lose its lubrication film and fail to provide adequate protection. At the same time, it should not be too thick, as this would cause unnecessary internal friction. Since viscosity is temperature-dependent and influenced by the quality of the oil (including particles and degradation products), selecting a lubricant with the correct viscosity requires specialised knowledge.

Viscosity describes the internal frictional forces between layers of fluid that move past one another. The higher the frictional forces, the harder it is for the fluid to flow, meaning it is more viscous – in other words, the higher its viscosity. With lower frictional forces, the fluid flows more easily, and the viscosity is lower. It is important to note that the viscosity of a lubricant is not a constant property; it changes depending on the temperature.

Even more important than viscosity is the viscosity index. This index indicates how much viscosity changes with temperature fluctuations. A lubricant with a low viscosity index will quickly become thinner or thicker with small temperature changes, resulting in limited performance across different operating temperatures. A higher viscosity index means the viscosity remains more stable over a broader temperature range, which is often desirable in variable conditions.

The viscosity of industrial lubricants is defined in ISO viscosity classifications, which measure viscosity at 40°C, a common operating temperature. For example, a lubricant with ISO VG 68 has a viscosity of 68 cSt at 40°C, with a margin of ±10%. There are also other standards, such as AGMA for industrial gear oils and SAE for automotive applications, with separate classifications for engine and gear oils.

Viscosity and the viscosity index are direct indicators of a lubricant's performance. Lubricants with low viscosity, similar to water, are often used at low temperatures, lower loads, and slower rotational speeds, such as in hydraulic systems. Lubricants with high viscosity are better suited for high-load applications, such as gearboxes and other heavily loaded industrial components.

It is important to understand that these classes are no indication of the quality of the oil as a lubricant, nor do they account for viscosity at temperatures other than 40°C. Therefore, determining the correct viscosity class is only part of the process; experience and knowledge are indispensable when making the right choice.