Over time, engine oil begins to degrade, it loses its effectiveness and can no longer provide adequate protection to the engine.
The oil can collect water formed in small amounts during heating and cooling of the engine, dust and dirt entering the engine through the air induction system. The oil can also react with an acid, formed by the reaction of water with other contaminants.
When driving, the contamination of the engine oil increases. The oil filter traps particles contained in the oil, but with time the oil itself begins to break down (oxidize or thicken). From this moment the oil can no longer perform to its optimum capability and so requires replacing.
The oil must be replaced before its contamination will reach a level that could lead to engine damage. Determining the level of contamination within the oil can be very difficult, as a result car manufacturers provide recommended oil change intervals. These recommendations depend on the year of manufacture of the car and make and model.