On August 7, 1996, at 1000 central daylight time, a Cessna 210-5, N8139Z, was substantially damaged during a forced landing near Muskogee, Oklahoma. The airplane was owned and operated by a private owner. The private pilot and his two passengers were not injured. Visual meteorological conditions prevailed for the personal flight conducted under Title 14 CFR Part 91. A weather briefing was obtained by the pilot prior to the flight and a flight plan was not filed. The flight originated from a private airstrip near Lonoke, Arkansas, at approximately 0800. According to the pilot, the engine started to run rough, and subsequently lost power while in cruise flight at 6,500 feet MSL. The pilot attempted to land at the Davis Airport, near Muskogee, Oklahoma; however, he was not able to reach the airport and landed in a field approximately one mile southeast of the airport. During the landing roll, the airplane impacted a fence and fence posts resulting in leading edge damage to one wing and structural damage to both wing struts. The engine was examined by the FAA inspector, the aircraft owner, and a representative from the engine manufacturer on August 28, 1996 at an FBO at Davis Field in Muskogee, Oklahoma. The contents of the fuel system were examined. The fuel in the tanks was "discolored" by an unknown substance and appeared yellow/straw in color. The fuel was also lacking the appearance and smell of 100LL fuel; however, no diesel or jet fuel was detected. New spark plugs were installed as the fuel tanks were drained and serviced with fresh 100LL fuel. The engine was started and operated normally through the normal operating range by the pilot/owner of the aircraft. The loss of engine power was attributed to the contaminated fuel found in the fuel tanks.