The pilot had leveled the airplane off for cruise flight when the engine's RPM dropped to approximately 1,500 and the oil pressure went to '0 psi.' A short time later, the engine vibrated and seized. The pilot attempted to maneuver the airplane during the forced landing to a nearby grass airstrip, but was unable to identify it due to the dark night condition. During the descent into what he thought was the airstrip, the airplane contacted the top of a tree. The pilot maintained control and landed in a bean field. During the landing roll, the nose gear became entangled in the high vegetation and collapsed. The airplane then nosed over. Engine disassembly revealed that the oil supply had been interrupted by the failure of the 78532 oil pump driven impeller. This loss of lubrication resulted in the engine sustaining severe damage to its rotating and reciprocating components. Examination of the engine's records indicated that the manufacturer's service bulletin #456 had not been complied with as outlined in AD 81-18-04R2, paragraph C, which required the removal of the 'sintered' iron impeller.