THE PILOT IN COMMAND PERFORMED A PREFLIGHT AND CHECKED THE OIL BY REMOVING THE DIP STICK/OIL FILLER CAP. AFTER 10 TO 15 MINUTES OF FLIGHT TIME, THE PILOT NOTICED A DECREASE IN THE TORQUE INDICATION AND ENGINE OIL PRESSURE. HE ELECTED TO RETURN TO DILLINGHAM AND ENROUTE THE ENGINE LOST MORE OIL PRESSURE AND 'COUGHED' TWICE AND THE PILOT FEATHERED THE PROPELLER AND LANDED ON THE TUNDRA. UPON LANDING THE AIRPLANE NOSED OVER. THE ON SITE INSPECTION SHOWED THAT THE LEFT, LOWER SIDE OF THE FUSELAGE WAS COVERED WITH OIL AND THE OIL DIP STICK/FILLER CAP WAS NOT INSTALLED IN THE FILLER NECK. IT WAS RESTING ON THE INSIDE OF THE UPPER COWLING BECAUSE THE AIRPLANE WAS UPSIDE DOWN. THE BRIEF SHOWS THAT THE PILOT WAS THE HOLDER OF AN AIRLINE TRANSPORT PILOT CERTIFICATE BECAUSE THE COMPUTER DATABASE PROGRAM WILL NOT ACCEPT A COMMERCIAL PILOT RATING WHILE SHOWING A SCHEDULED 14 CFR 135, PASSENGER/CARGO TYPE OF OPERATION. THE PILOT IS ONLY THE HOLDER OF A COMMERCIAL PILOT CERTIFICATE.