On April 12, 1999, at 0915 hours Pacific daylight time, an experimental Vans RV-4, N6420E, was substantially damaged during an emergency off-airport landing following loss of engine power during cruise flight near Red Bluff, California. The commercial pilot, the sole occupant, was not injured. The amateur built aircraft departed from Rio Vista, California, at 0830. Visual meteorological conditions prevailed for the personal flight, which was destined for Medford, Oregon. The aircraft was operated by the pilot/owner under 14 CFR Part 91. No flight plan was filed. The Safety Board received notification of the accident on April 23, 1999. According to the pilot, the engine stopped (seized) after a seam of the automotive-type oil cooler opened up in-flight, resulting in loss of the engine oil supply and oil starvation of the engine. The pilot attempted to make a forced landing in a field, however, the right main landing gear entered a hole and the aircraft "cartwheeled."