What is oil viscosity?

Probably the most important property of oil lubricant is the absolute viscosity, which is a measure of the force required to move one layer of the oil film over the other. If the viscosity is too low, a protecting oil film is not formed between the parts. With high viscosity too much power is required to shear the oil film, and the flow of oil through the engine is retarded. Viscosity tends to decrease as temperature increases. Viscosity index (VI) is a number that indicates the resistance of an oil to changes in viscosity with temperature. The smaller the change in viscosity with temperature, the higher the viscosity index of the oil.