Incorrect oil as turbo said or low on oil are the most common causes. If the oil is not changed at correct intervals, it can also go out of grade, ie, a 10W50 can become a 5W30. I always listen to number 3 with a stethescope and give the engine a good rev before I do any engine work on one. If there is slight wear, you can here a slight rumble. The oil pump and the crank are not the best design's ever.
John