What was found
AFTER ENGINE SHUTDOWN AT GATE, WITH THE APU PROVIDING PNEUMATIC POWER FOR AIR CONDITIONING, CABIN BEGAN TO FILL WITH SMOKE AND STRONG BURNING OIL ODOR. FLIGHT CREW SHUTDOWN APU, SMOKE AND ODOR PROMPTLY DISSIPATED. DISCOVERED AN EXTERNAL OIL LEAK FROM APU AROUND THE GENERATOR DRIVE PAD AREA. APU REPLACED AND DUE TO RESIDUAL OIL ODOR IN CABIN, BOTH RECIRCULATION FAN FILTERS WERE REPLACED. NO ADDITIONAL REPORTS OF SMOKE NOTED ON SUBSEQUENT FLIGHTS.