On a cold morning, it is the high viscosity or stiff-ness of the lube oil that makes an automobile engine difficult to start. The viscosity must always be high enough to keep a good oil film between the moving parts. Otherwise, friction will increase, resulting in power loss and rapid wear on the parts. Oils are graded by their viscosities at a certain temperature. The right grade of oil, therefore, means oil of the proper viscosity.
Every oil has a viscosity index based on the slope of the temperature-viscosity curve.
The viscosity index depends on the rate of change in viscosity of a given oil with a change in temperature. If you are using an oil with a high viscosity index, its viscosity or body will change less when the temperature of the engine increases. The pour point of an oil is the lowest temperature at which the oil will barely flow from a container. At a temperature below the pour point, oil congeals or solidifies. Lube oils used in cold weather operations must have a low pour point.