What was found
FLIGHT 8974, PRECAUTIONARY LANDING AND AN EMERGENCY DECLARED EN-ROUTE DUE TO A FLUCTUATING OIL PRESSURE GAUGE. PILOT ADVISED OIL PRESSURE GAUGE FLUCTUATED +/- 4 DURING FLIGHT, THEN LEVELED OFF AT 5,000 MSL. ACFT LOST SOME OIL PRESSURE, BUT MAINTAINED IN THE GREEN ARC. ON LANDING AND ROLL OUT, OIL PRESSURE GAUGE DECREASED TO APPROX 65 PSI AND IT WAS IN THE YELLOW CAUTION ZONE ARC. MX R & R OIL PRESSURE GAUGE IAW MM 79-30-00. REMOVED AND CLEANED AND REINSTALLED ENGINE OIL FILTER IAW MM 79-20-00. ENGINE RUN OPS AND LEAK CHECKED GOOD - OIL PRESSURE IN THE GREEN ARC ALL RANGES.