Engine oil is the lifeblood of the engine. The oil resides in the oil pan, which is under the car attached to the bottom of the engine. All internal (moving) parts of the engine need to be lubricated by the engine oil. Inadequate lubrication will cause the parts to wear out faster and eventually lead to engine failure. An oil filter keeps the oil clean and free of debris. If the filter is not replaced on a regular basis, it will get clogged and will not be able to pass oil into the engine.