Getting regular engine oil changes as part of your vehicle maintenance routine is one of the best things you can do for the long life of your car. Without it, excessive friction damages the moving parts of your engine and reduces performance. Fortunately, modern automakers design their engines with controls that influence factors like gas mileage and emissions. Engine oil and oil pressure power these controls.
The primary role of engine oil is to lubricate the rotating or sliding surfaces between components in your engine's crankcase, including the main bearings and big-end bearings that support the pistons, as well as connecting rods that connect the pistons to the crankshaft. Oil also helps cool these parts, protecting them from overheating.
Oil degrades over time and limits its ability to lubricate the moving parts in your engine, but it still has many other important functions. The most obvious signs of degradation are an unusually dark color and thicker texture, as well as a burnt smell. In extreme cases, when you see a dramatic drop in the oil level without evidence of leakage, it might suggest that your engine is forming sludge and no longer effectively lubricating the moving parts.
Engine oil also helps protect the metal parts of your engine from corrosion, particularly at elevated temperatures, and it contains detergents to clean deposited contaminants off internal surfaces. It is also able to absorb soot from combustion, which a filter would normally trap on the surface of your engine.