After takeoff, during cruise flight, the engine sustained a short duration of oil pressure fluctuations followed by a period of stabilized engine oil pressure.  It then sustained a complete loss of oil pressure with a subsequent catastrophic engine failure. The pilot flying transferred control to the more experienced pilot-rated passenger, who maneuvered the airplane for a forced landing to a road. While on final approach to the road, the airplane’s left wing struck a tree and then the airplane then struck two street signs during the landing roll. The airplane came to rest in a ditch with substantial damage to one elevator.Postaccident examination of the engine revealed internal catastrophic failure consistent with oil starvation. Data from an engine monitor showed that the oil pressure during the accident flight was in the normal range before suddenly decreasing to 0 psi. The data also showed that about 11 days and 6 engine operational hours before the accident flight, while taxiing for a flight following installation of an engine-driven standby alternator, the oil pressure dropped to about 9 psi, and then dropped to about 3 psi during the takeoff roll. Both values were below the minimum 10 psi specified in the airplane’s Pilot’s Operating Handbook and Federal Aviation Administration (FAA) Approved Airplane Flight Manual. The near-0 psi actual oil pressure was only detected by the pilots after becoming airborne during that post-maintenance flight, resulting in an immediate return and uneventful landing. After landing, oil was found on the bottom of the fuselage and a large pool of oil was found on the taxiway where they had performed the first engine run-up to test the recently installed standby alternator. The maintenance facility then examined the engine and found that the standby alternator had not been installed (torqued) properly, resulting in the oil leak. Just over 4 quarts of oil remained in the engine. Page 2 of 10 ERA23LA086The standby alternator was replaced and installed properly. Oil was added to the engine, and a new oil filter was installed for an intended flight to another maintenance facility for an annual inspection. No contaminant was noted in the oil filter element what was removed from the engine. Normal engine oil pressure was noted for the majority of the flight to the maintenance facility except for one period when it temporarily dropped below 30 psi. As part of the subsequent annual inspection, the oil was changed again, and light metal debris was noted in the oil filter element. A new oil filter was installed, engine oil was added, and the airplane was then flown uneventfully on a short repositioning flight after the annual inspection.  Normal oil pressure was noted during the repositioning flight. In total, the airplane was operated for about 6 hours between the installation of the standby alternator and the accident flight.Based on the available information, it is unlikely that the lubrication distress damage to the engine was initiated during the accident flight. Rather, the previous prolonged engine operation at high rpm with near 0 psi actual oil pressure started internal damage to the connecting rod bearings that was undetected by two maintenance facilities. Had the two maintenance facilities required a disassembly inspection of the engine in accordance with guidance contained in the engine manufacturer’s Standard Practice Maintenance Manual, it is likely that the internal damage to the connecting rod bearings would have been detected and repaired before the engine ultimately failed catastrophically during the accident flight.