Lubricating oil, sometimes simply called lubricant/lube, is a class of oils used to reduce the friction, heat, and wear between mechanical components that are in contact with each other. Lubricating oil is used in motorized vehicles, where it is known specifically as motor oil and transmission fluid.
SAE 40 is synthetic oil derived from natural or crude oil. Here, SAE stands for Society of Automotive Engineers and 40 refers to viscosity of the oil. In layman's term we can say 40 is thickness of oil. More is the thickness, lesser is the contact between metals, which increases the life of engine.
20W40 is a commonly used motor oil grade. ... The letter W indicates winter and at last the number “40” stands for the viscosity of oil when the engine is operating or hot. In simple terms viscosity means the thickness of the liquid, here oil. Higher the number, thicker is the liquid and therefore harder to flow.