While on a cross country flight, the pilot elected to make an off airport landing to an open field due to a fluctuating oil pressure gauge. The pilot said that the "wing tip hit the wheat which rotated the aircraft as the landing gear touched the ground." The pilot reported no mechanical malfunctions with the airplane reference the flight. An examination of the wreckage revealed no anomalies that would have precluded operation of the airplane. The pilot's safety recommendation was, "Think about the landing not saving the engine. Land in the center of the field not the edge ... ."