One of the most important properties of oil is its viscosity. The viscosity of any oil becomes less as the temperature rises so that it is desirable that a machine should retain a reasonable viscosity even at high temperature.
Meanwhile, thick or viscous oil is a disadvantage at low temperature. It makes starting difficult for an engine because it will not circulate rapidly enough when an engine is started.
Various physical units have been used to describe dynamic viscosities. The new metric systems have the units of milli-pascal second. The viscosity of water at room temperature is about 1mpa. This provides a direct feeling for the viscosities of other substances by comparison with water. Maker of motor vehicles often recommend the use of an oil of lower viscosity for cold season (raining or harmattan season use).
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From @neyobuss