On June 21, 2015, about 1130 central daylight time, the pilot of a Mooney M20K, N1168Z, made a forced landing in a wheat field 5 miles south of Sheboygan (KSBM), Wisconsin. The pilot, the sole occupant on board, was not injured. The airplane was substantially damaged. The airplane was registered to and operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91 as a personal flight. Instrument meteorological conditions prevailed, and an instrument flight rules flight plan had been filed and activated. The flight originated from KSBM at 1113 and was destined for Elkhart (KEKM), Indiana.Shortly after airplane departed, the pilot advised he was returning to KSBM due to low oil pressure. The airplane lost engine power and the pilot made a forced landing in a wheat field five miles south of the airport. A Federal Aviation Administration (FAA) airworthiness inspector who examined the airplane reported finding no oil in the crankcase. He reported the rear bulkhead was wrinkled and the empennage was twisted. On July 1, 2015, an FAA airworthiness inspector examined the engine and turbocharger. He reported the turbocharger was dry and did not appear to be at fault. On August 17, the FAA airworthiness inspector examined the engine again at a teardown facility in Poplar Grove, Wisconsin. He reported finding evidence of pre-ignition in the number 5 cylinder that appeared to have been caused by failed spark plugs. The top and sides of the piston head were eroded, allowing exhaust gases to pressurize the crankcase and force engine oil out of the breather tube.