At approximately 1600 Pacific time, a Cessna 182J was damaged in a forced landing after losing engine power about four miles east of Salem. The pilot was not injured. The pilot said he was cruising at 11,500 feet when the engine slowly started losing power. Eventually the engine lost enough power that the pilot was unable to hold altitude and he found it necessary to attempt a forced landing in an open field. The touchdown was uneventful but the aircraft encountered rough terrain during the landing roll. Initial inspection revealed there were holes in the crankcase and most of the oil had escaped.