On August 1, 2008, about 1915 central daylight time, a Cessna 172RG, N6520R, piloted by a private pilot, sustained substantial damage on impact with terrain following an in-flight loss of engine power near the Tomahawk Regional Airport (TKV), Tomahawk, Wisconsin. The personal flight was operating under 14 CFR Part 91. Visual meteorological conditions prevailed at the time of the accident. No flight plan was on file. The pilot and a passenger sustained minor injuries. The local flight originated from TKV about 1900. The pilot reported that he and his passenger noticed light smoke in the cabin during cruise at 2,500 feet above mean sea level (MSL) about five miles east-northeast of TKV. He turned left to return to TKV and opened the windows to vent the cabin. He stated that all the airplane's instruments indicated normal. After the turn, oil began to accumulate on the windshield, which obscured his forward visibility. He made a five mile straight-in approach using the airplane's global positioning system. He said that he attempted to maintain 2,500 feet MSL and began to lose engine power about three miles from TKV. The engine "sputtered then froze" about two miles from TKV. The pilot made a forced landing on a field that the passenger saw out of his window. The pilot said that the airplane came to rest at the end of that field where it impacted trees at about 30 knots. Subsequent to the accident, the propeller and starter ring gear were removed. The oil plug on the nose of the engine case that covers the governor idler gear shaft was not in place and it was found in the cowling. Safety wire that had secured the plug was observed attached to engine case with a separation at its plug end. The pilot's safety recommendation stated he had accumulated 23 hours of simulated instrument training. He said, "Without the instrument time I am certain the loss of all forward visibility would have been unnerving."