What was found
(CAN) 30 MINUTES AFTER TAKEOFF #1 OIL QUANTITY STARTED A GRADUAL DECREASE. POWER REDUCED ON REACHING 2 GALLONS. ENGINE SHUT DOWN AT ZERO QUANTITY, PRESSURE AND TEMPERATURE NORMAL. ON PREVIOUS LEG OIL PRESSURE WENT TO ZERO JUST BEFORE TOUCHDOWN. ALL OTHER INDICATIONS WERE NORMAL. SUSPECT INSTRUMENT FAILURE, OIL ADDED. FOUND VENT LINE MISSING AND VENT PORT NOT CAPPED. REPLACED.