En route from Mountain View, Missouri, to Hopkinsville, Kentucky, the airplane's engine oil pressure dropped from 78 psi to 70 psi. A precautionary landing was made and a visual check of the airplane was accomplished. The oil pressure was still indicating a safe pressure, so the pilot continued the flight to Hopkinsville. Approximately 5 nautical miles outside of Hopkinsville, the engine's oil pressure dropped to zero and the engine lost partial power. The pilot was making a forced landing on a highway when the airplane struck a single guy wire stretched across the highway. The airplane impacted the ground, nosed over and slid approximately 90 feet before coming to rest upright. An engine inspection revealed an excessive amount of oil in the exhaust stack and port of the number 4 cylinder. Examination of the number 4 cylinder revealed the following: a broken oil ring, the number 2 ring frozen in the groove of the piston, and one of the piston plugs was worn with excessive wear on the piston plug's hole.