What was found
(CAN) SHORTLY AFTER TAKEOFF THE PILOT NOTICED THE #1 ENGINE OIL QUANTITY GAUGE WAS DROPPING. THE AIRCRAFT RETURNED TO ITS POINT OF DEPARTURE. POST FLIGHT INSPECTION REVEALED THE PRESSURE OIL FILTER GASKET WAS MISSING A SECTION. THE GASKET WAS REPLACED AND THE OIL FILTER COVER NUTS (P/N AN3850) WERE REPLACED AND TORQUED. ENGINE WAS GROUND RUN SERVICEABLE AND THE AIRCRAFT WAS RETURNED TO SERVICE.