On May 4, 2000, at 0830 Pacific daylight time, a Gulfstream-Schweizer G-164B (Ag Cat), N8130K, was substantially damaged during an off-airport forced landing, following a loss of engine power 2 miles northwest of Ephrata, Washington. The commercial pilot was not injured. Visual meteorological conditions prevailed and no flight plan was filed for the local 14CFR137 aerial application flight. The flight originated from a service airstrip approximately 15 minutes prior to the accident. The pilot reported that while on a routine aerial application flight, engine oil began to appear on the aircraft's windscreen. Shortly thereafter, approximately 2 miles northwest of Ephrata, the aircraft's engine began to "backfire and eventually quit". The pilot executed a forced landing in an open field of spring wheat. During the landing roll-out, the aircraft encountered soft terrain and nosed over. The aircraft sustained substantial damage to the wings and empennage. An FAA Inspector from the Spokane, Washington, Flight Standards District Office, inspected the aircraft after the accident and reported that the exhaust valve push rod on the number three cylinder had failed.