What was found
(CAN) IN FLIGHT, PILOT NOTICED THE OIL PRESSURE GAUGE FLUCTUATING. PILOT TURNED AROUND AND RETURNED TO DEPARTURE. WHILE RETURNING, OIL PRESSURE DROPPED TO 0 AND OIL TEMP CLIMBED TO RED LINE. PILOT SHUTDOWN ENG AND GLIDED BACK TO AIRPORT. ON TROUBLESHOOTING, CAUSE OF OIL LEAK, B-NUT ON PROP GOVERNOR LINE WAS FOUND TO BE LOOSE. ACFT HAD UNDERGONE MX JUST PRIOR TO THIS FLIGHT. THIS B-NUT WAS NOT TOUCHED IN THE MX FUNCTION. ON TALKING WITH STAFF, ONE MUST NOT ASUME BECAUSE A LINE OR HOSE IS NOT LEAKING THAT IT IS TIGHT. ON ALL INSP FROM NOW ON, WILL BE CHECK TIGHTENING ALL FLUID LINES. THIS LESSON COST AN EARLY ENGINE CHANGE.