DURING TAXI IN; MD80 EVACUATES ALL PAX ON RAMP FOLLOWING SEVERE SMOKE INCIDENT. SMOKE FOUND TO HAVE BEEN THE RESULT OF AN APU OIL LEAK INGESTED BY THE AIR CONDITIONING PACKS.

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AFTER CLEARING RWY 28L AND TAXIING ONTO TXWY A; AFTER LNDG CHKLIST CALLED FOR AND COMPLETED BY FO. THIS INCLUDED START UP OF THE APU AND SELECTING BLEED AIR FROM IT. SOMETIME AFTER THIS; FLT ATTENDANTS CALLED ON INTERPHONE ACCOMPANIED BY BANGING ON THE COCKPIT DOOR. FO ANSWERED INTERPHONE AND I LISTENED IN. THEY ADVISED SMOKE OF UNKNOWN ORIGIN IN CABIN. I TURNED OFF AIR CONDITIONING PACKS AND APU BLEED AIR AND ASKED IF THEY COULD STAND BY AS WE WERE ALMOST TO THE GATE. ONE STATED THAT WE MUST GET OUT NOW! IN THE BACKGROUND I COULD HEAR LAV SMOKE ALARM CHIMES OVER THE INTERPHONE. JUST ABOUT THE SAME TIME SMOKE APPEARED IN THE COCKPIT COMING THROUGH THE COCKPIT DOOR. I STOPPED THE ACFT ON TXWY H; SET BRAKES; ORDERED THE CABIN TO EVACUATE; DONNED OXYGEN MASK AND CALLED FOR EVACUATION CHKLIST. SOMEWHERE IN THE MELEE; THE FO NOTIFIED GND OF WHAT WAS TAKING PLACE AND TO DISPATCH EMER EQUIP. AFTER COMPLETION OF THE CHKLIST; I ORDERED THE FO TO EVACUATE AND I WENT TO CABIN FOR PAX CHK. ONLY ONE FLT ATTENDANT REMAINED ON BOARD AND WE BOTH WENT DOWN THE L1 SLIDE. NO INJURIES WERE RPTED TO ME AND 140 PAX WERE EVACUATED IN ABOUT ONE MINUTE. TEXTBOOK ALMOST. POST FLT INSPECTION REVEALED AN OIL LEAK IN THE APU COMPARTMENT THAT WAS THEN INGESTED INTO THE AIR CONDITIONING SYSTEM CAUSING THE SMOKE. TWO SYSTEMS INDICATIONS ON THE MD80 BOTHER ME AND WOULD HAVE ASSISTED IN TROUBLE SHOOTING AND DECISION MAKING. ONE IS THAT THERE IS NO CAUTION LIGHT TELLING US THAT A POSSIBLE PACK OVERHEAT HAS TRIPPED A PACK (WHEN THEY GOT HOT IT CAN CAUSE SMOKE IN CABIN). THE SECOND IS THAT LAV SMOKE DETECTORS DO NOT SHOW A COCKPIT WARNING. THESE ITEMS MAY HAVE BEEN AN EARLIER WARNING/TROUBLE SHOOTING AIDE FOR US IN THE COCKPIT.