What is Viscosity?
Viscosity is a measure of a liquid's ability to resist flow. Low viscosity means the liquid is thin and easily poured. High viscosity means the liquid is thick and not easily poured. Oil must have the correct viscosity to protect and lubricate all engine parts. If the viscosity is too low (thin), the oil film between moving parts will be squeezed and insufficient to prevent part wear. Alternatively, if the oil viscosity is too high (thick), the oil will not readily flow to all parts to lubricate them and part wear will also occur. This is particularly harmful on a cold engine since thick oil takes longer than thin oil to circulate to distant parts.
Generally, cold temperatures increase oil viscosity (make it thicker) and hot temperatures decrease oil viscosity (make it thinner). However, extremely high temperatures cause evaporation of certain chemicals and thus cause the oil to permanently thicken after it cools. High viscosity oils are sometimes called heavyweight oils. Low viscosity oils are also called lightweight oils.