The aircraft was on the first flight since the completion of a 100 hourly maintenance inspection. The pilot reported that the engine oil pressure decreased to an unsafe level, 16 minutes into the flight, so he carried out a precautionary landing on a short disused strip. Towards the end of the landing roll, the pilot attempted to groundloop the aircraft to avoid rolling off the end of the strip. During the groundloop, the right main gear collapsed and the rear fuselage was distorted. The investigation revealed that one of the nuts securing the vacuum pump was loose and the gasket had failed. The loose nut had allowed engine oil under pressure, from an adjacent oil gallery, to force past the gasket and pump overboard. The vacuum pump had been fitted during the last maintenance inspection and a gasket of incorrect material had been fitted. The current Visual Terminal Chart for the area shows an aerodrome landmark at the location of the incident. This strip did not meet the Authorised Landing Area standards and the identifier on the chart clearly indicated that the status and serviceability of the strip were unknown. The surface condition and length of the strip were unsuitable for a safe precautionary landing, but the pilot had no means of ascertaining this prior to touchdown. The presence of the symbol on the map may have suggested to the pilot that there was no need to look for a more suitable landing area.