DURING FLIGHT, THE ENGINE SURGED AND THE PROPELLER SPEED WENT TO 3000 RPM. THE PILOT RETARDED THE PROP CONTROL, AND MAINTAINED 2400 RPM. HE PROCEEDED TO THE GASTONIA AIRPORT AND LANDED WITHOUT FURTHER INCIDENT. A GROUND RUN-UP WAS MADE AND ALL INSTRUMENTS AND CONTROLS CHECKED NORMAL. HOWEVER, THE PILOT DID NOT SHUTDOWN THE ENGINE OR EXIT THE AIRCRAFT. A WITNESS OBSERVED OIL ON THE AIRCRAFT, BUT BEFORE HE GOT TO THE PLANE, THE PILOT TOOK- OFF. THE WITNESS SAW OIL ON THE RAMP WHERE THE AIRCRAFT HAD STOPPED. LATER AT 1600 FEET MSL, THE ENGINE WENT TO 3000 RPM AND THE PILOT NOTED A DROP IN OIL PRESSURE. IMMEDIATELY, THE ENGINE BEGAN TO KNOCK AND LOSE POWER. THE LEFT GEAR FAILED DURING LANDING IN A SMALL PASTURE. INVESTIGATION REVEALED OIL HAD ESCAPED FROM THE BASE OF THE OIL FILTER, WHERE THE OIL SEAL WAS OUT OF ITS GROOVE. A 100 HOUR INSPECTION HAD BEEN PERFORMED ON THE PREVIOUS DAY.