The airplane was substantially damaged when it struck a road sign at about 1607 Mountain time during a forced landing. Visual conditions prevailed; the solo Private pilot was not injured. The pilot subsequently reported that, while in cruise flight, the engine manifold pressure dropped followed by a loss of oil pressure. The pilot elected to divert to Artesia, N.M., when the airplanes engine seized. The pilot performed a forced landing to a highway. During the emergency landing, the airplanes right wing struck a road sign, causing substantial damage to the airplane.