Motor oil breaks down over time. When it breaks down, it loses its effectiveness and can no longer properly protect your engine. In addition to lubricating an engine's moving parts, motor oil is designed to carry combustion byproducts away from the pistons and cylinders. It is designed to deal with the small amounts of water that form as the engine heats and cools, and to collect the dirt and dust that enter the engine through the air-intake system. It also handles acids that are formed by the reaction between water and other contaminants. Sometimes there are even fuel leaks (fuel dilution) or coolant leaks that get into the oil system. As a car is driven, the level of contamination in the motor oil constantly increases. The oil filter removes particles as the oil passes through the filter, but over time an oil's additives are used up and the oil itself can start to degrade (oxidize or thicken). At that point, the oil can no longer do its job and must be changed.
The rate at which contamination and additive depletion occurs depends on many variables. One of these is driving conditions, which vary greatly and have a direct effect on the useful life of the oil. Other factors include the precision of ignition, fuel injection or carburetion adjustments, air cleaner service, and the general mechanical condition of the engine.
Oil should be changed before the contamination level reaches the point where engine damage can result. Because it is difficult for the individual motorist to determine when the contamination level is too high, automobile manufacturers provide recommended oil change intervals. These change recommendations vary by model year and manufacturer. Recommended intervals and mileage limits also vary with the type of service under which a car operates. More frequent oil changes are recommended for severe service.