What was found
CABIN SMOKE: WHILE ON GROUND WITH APU ON, A FINE MIST WITH AN ODOR OF ENGINE OIL BEGAN TO APPEAR & THICKENED UNTIL BLEED AIR TURNED OFF. COMPLIED WITH TC-68A-21-181006M ENVIRON CONTROL SYS BLEED DUCT BURN PROCEDURE & NOTED SMOKE WHILE DOING PROCEDURE. FOUND APU OIL LEVEL LOW NO OIL ON APU OIL DIP STICK. NOTICED OIL ON APU INLET DRAIN LINES. MCC REQUEST TO BOROSCOPE APU & LOOK FOR INTERNAL OIL LEAKS.PERFORMED BORESCOPE OF APU. COMPLIED WITH ENVIRON CONTROL SYSTEM BLEED DUCT BURN PROCEDURE WITH ENGINES RUNNING ONLY & NO SMOKE NOTED IN COCKPIT & CABIN. REMOVED & REPLACED HEPA FILTER WITH A CUSTOMER SUPPLIED NEW PART PER NJA TASK TC-68A-21-181006M & AMM 21-70-10.