Viscosity is the flowability of liquids. A liquid that is relatively viscous can be described as viscous. Water, for example, has a low viscosity compared to honey. In this example, honey has a higher viscosity at the same temperature.

A high-quality lubricant maintains its viscosity under different temperature and operating conditions over a longer period of time.

Most of today's engine oils are multigrade oils, e.g. "5W-40", i.e. they retain their lubricating properties at both high and low temperatures. The first number, followed by a W for winter, indicates the viscosity at low temperatures. The second, higher number indicates the viscosity at high temperatures. A multigrade oil minimizes viscosity differences during temperature fluctuations and can therefore be used at different temperatures, e.g. in summer and winter.