At 0720 Central time the aircraft collided with soft terrain during an off-airport forced landing. The Private pilot was not injured, although the airplane was substantially damaged. The flight was operating in visual conditions. The pilot subsequently reported being in cruise flight at 9000 feet when he noticed lower-than-normal oil pressure. He headed toward the closest airport as the oil pressure, oil temperature and manifold pressure all decreased. After determining he would not make it to an airport, the pilot selected an open field in which to land. When the airplane touched down, its landing gear sank into the muddy terrain. According to the pilot, the airplane rolled about 25 feet prior to coming to a stop. Investigation revealed a hole in the crankcase below the #4 cylinder.
