What was found
ACFT WAS IN CRUISE FLT, PILOT NOTICED OIL PRESSURE DEMINISHING, PILOT INITIATED LANDING. ONCE ACFT WAS ON THE GROUND PILOT NOTICED OIL PRESSURE AT FLUCTUATING FROM ZERO TO 30 PSI, AND ALSO NOTICED SMOKE COMING FROM ENGINE. ENGINE WAS SHUTDOWN AND MX WAS CALLED OUT. UPON ARRIVAL, TECH NOTICED OIL ON BOTH SIDES OF MID FUSELAGE. UPON FURTHER INSPECTION, WITH ENGINE COWLING REMOVED, NOTICED THAT THE ENGINE OIL FILTER CAP WAS NOT FULLY SEATED. PROCEEDED TO FURTHER INSPECT AND FOUND THAT 1 OF THE STUDS THAT SECURES THE CAP DOWN HAD WORN THREADS.