AFTER NOTING LOW OIL PRESSURE; A C421B INSTRUCTOR PLT ON APCH TO JYO CANCELED IFR AND SELECTED 1200 IN HIS XPONDER IN VIOLATION OF DC ADIZ PROCS.

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I WAS THE PIC AND SERVING AS THE CFII IN A C421B. THERE WERE NO PAX ON BOARD. THE FLT WAS FROM TYS TO JYO. THE FLT WAS OPERATED UNDER IFR. THE FLT WAS FLOWN AT FL210. I WAS PROVIDING DUAL TRAINING TO THE L SEAT PLT IN ORDER TO COMPLY WITH INSURANCE REQUIREMENTS. IN KNOXVILLE; WE NOTICED THAT THE OIL LEVEL ON THE L ENG WAS LOW AND WE NOTICED SOME OIL ON THE FLAPS AND OTHER UNDER WING AREAS. WE ADDED OIL AND DECIDED TO STOP IN JYO -- A 2 HR FLT -- TO CHK ON THE OIL STATUS. FINAL DEST WAS CONCORD; NH. APPROX 45 MINS FROM JYO I NOTICE THE MANIFOLD PRESSURE AND FUEL FLOW WAS FLUCTUATING ON THE L ENG AND OIL PRESSURE WAS DECREASING. THE DSCNT WAS NORMAL WHILE FLYING A GPS DIRECT COURSE WITH SOME ATC VECTORS FOR TFC. APPROX 6 MINS FROM JYO THE OIL PRESSURE WAS AT THE BOTTOM OF THE GREEN ARC WITH BOTH PLTS FOCUSING ON THE POSSIBLE NEED TO SECURE AN ENG. THE CREW WAS IN VMC AND HAD JYO IN SIGHT AND SO ADVISED ATC TO CANCEL THE IFR FLT PLAN. WE WERE TOLD TO PROCEED DIRECT TO LEESBURG AND TO SWITCH TO THE ARPT'S ADVISORY FREQ. REACTING FROM WHAT IS 40 YRS OF FLYING (INSTINCT) I IMMEDIATELY PRESSED THE GARMIN GTX330 XPONDER'S VFR (CODE 1200) BUTTON. THIS WAS A VIOLATION OF THE DC ADIZ. I SHOULD HAVE KEPT THE ASSIGNED IFR CODE IN THE XPONDER. I AM FROM THE DALLAS; TX; AREA AND DID NOT KNOW THE REQUIREMENT TO NOT SWITCH TO CODE 1200 WHEN CANCELING IFR AT A NON TWR ARPT WHICH IS THE STANDARD PROC EVERYWHERE ELSE. MY INSTANT REACTION TO CHANGE TO THE VFR CODE WAS A HABIT FROM 40 YRS OF SAFE FLYING. UPON LNDG; WE CALLED ATC AS HAD BEEN REQUESTED BY ATL TO THE JYO FBO. I WILL BE ATTENDING AN AOPA FLT INSTRUCTOR REFRESHER COURSE SOON. AIRSPACE AND THE WASHINGTON; DC ADIZ WILL BE REVIEWED. THE MISTAKE WAS THE RESULT OF THE PIC NOT BEING FAMILIAR WITH THE LCL LEESBURG ATC PROCS AND THE HABIT TO COMPLY WITH WHAT IS NORMALLY THE PROPER PROC WHEN CANCELING IFR AT A NON TWR ARPT.