Viscosity the use of oil which basically have minimal effect on the change of viscosity with respect to the changes of surrounding temperature.
In lubrication, viscosity is the most important property or characteristic of an oil. Viscosity means the “thickness” of an oil, where specific application an oil requires sufficient viscosity to maintain proper lubrication. However, viscosity is most affected by the operating temperature of the surrounding areas, hence, you need to maintain proper viscosity so you protect the surface of friction and wear.
Therefore, an oil to effectively lubricate, it has to maintain enough film thickness all throughout its operating range of temperature, cold and hot condition. By using a multi-grade oil, it acts to lubricate properly for both hot and cold conditions, in other words, wider range of operating temperature.
It is unlike single or mono-grade oil which does not have the flexibility to lubricate at wider range of temperature.