What was found
(CAN) COPILOT NOTICED NR 2 ENGINE OIL PRESSURE IND FLUCTUATING THEN FOLLOWED BY ILLUMINATION OF ENGINE OIL PRESSURE WARNING LIGHT. PILOT BROUGHT ENGINE BACK TO FLT IDLE WHERE ENG OIL PRESSURE STABILIZED AT 38 PSI. ON FINAL APPROACH, ENG OIL PRESSURE DROPPED TO 20 PSI AND LANDED WITH SINGLE ENG. AN INSP OF SUBJECT ENG REVEALED THAT OIL FILTER COVER ATTACHMENT STUD HAD PULLED OUT OF ACCESSORY GEARBOX CAUSING FILTER COVER TO DROP DOWN SLIGHTLY MAKING THE O-RING SEAL INEFFECTIVE. ENG GEARBOX AND STUD REPLACED IAW MFGS INSTRUCTIONS, ENG OIL FILTER INSPECTED, CLEANED, AND REINST. ENG SERVICED WITH OIL AND GROUND RUN FOR LEAKS. ADDITIONALLY, VIBRATION SWEEP CARRIED OUT.