WHILE IN LOW LEVEL CRUISE AFTER TAKEOFF ON A AERIAL APPLICATION FLIGHT, A MECHANICAL LOSS OF POWER OCCURRED. THE PILOT CONDUCTED A FORCED LANDING ON A HILLSIDE IN MOUNTAINOUS/HILLY TERRAIN. A POST IMPACT FIRE DESTROYED THE AIRCRAFT. THE POWER LOSS WAS DUE TO LOSS OF ENGINE OIL AS A RESULT OF IMPROPER MAINTENANCE.