The viscosity of an oil is the first and most important consideration when selecting an oil for a specific application. For the most effective lubrication, the viscosity must conform to the speed, load and temperature conditions of the lubricated parts.

A significant factor of viscosity is the ability of an oil to resist shearing or the "tearing away of one plane of lubricant from another" during the lubrication function. Under certain conditions, such as shock loads, continuous heavy loading, extremely high temperatures and/or critically low (thin) viscosity, lubricants may not remain in their normal hydrodynamic film state and damage starts to occur.

Viscosity affects heat generation in cylinders, bearings and gear sets related to an oil's internal friction. It governs the sealing effect of oils and the rate of oil consumption, as well as determines the ease with which machines may be started or operated under varying temperature conditions, particularly in cold climates.
 

Refer to your vehicle service manual for correct lubricant requirements and service intervals.