Five minutes after departure and during normal cruise flight, the certified flight instructor noticed a prop overspeed. He immediately pulled back on the prop control to reduce engine R.P.M. and instructed the student, a private pilot, to divert to the airplane's home base. Two minutes after the overspeed, the engine 'sputtered' for 5 seconds and seized. The instructor took control of the airplane and executed a forced landing on a hillside. The airplane brushed small branches at the top of a tree and collided with the terrain. The pilot stated that he 'used all the airspeed to reach a suitable landing spot', and could not flare the airplane to cushion the landing. An examination of the engine revealed evidence of an oil exhaustion. The oil quick drain was found to be in the open and locked position. An examination of records revealed that the drain was the incorrect part. There was an AD concerning the incorrect oil drain being opened by contact with the landing gear during retraction.