What was found
(CAN) PILOT REPORTED THE NR 1 ENGINE OIL AND TORQUE PRESSURE DROPPED TO ZERO. NR 1 ENGINE PROP RPM FROM 1900 TO 1500. N1 AND ITT WERE NORMAL. THE ENGINE WAS SHUTDOWN WITHIN MINUTES. POST- FLIGHT INSPECTION FOUND NR 1 ENGINE OIL PRESSURE RELIEF VALVE TO BE UNSECURED. THE RELIEF VALVE WAS RESECURED. ALL FILTERS WERE INSPECTED AND CLEANED. THE ENGINE WAS RUN-UP AND THE VALVE AND FILTERS WERE CHECKED SERVICEABLE.