It neutralizes acids which can corrode metal. Oil loses its lubricating qualities as its additives wear out and it picks up dirt and other abrasive material. Black crankcase oil does not indicate that an oil change is due. Check the operator's manual for the manufacturer's recommended oil change schedule to insure that the oil is changed before the additives wear out. After each 25 hours of engine use, the crankcase oil should be drained while the engine is warm.
Hot oil flows easily and carries the contaminants with it.
Start the engine, and let it run until the normal operating temperature is obtained. Turn off the engine and remove the drain plug from the oil pan. Drain the oil for at least five minutes. Before removing the oil filter, clean dirt off the filter and surrounding area. If the filter is self-contained, remove it with an oil filter wrench.