A DC9-30 ON INITIAL CLB AT 6000 FT DECLARED AN EMER AND DIVERTED DUE TO SMOKE IN THE CABIN CAUSED BY OIL SEEPAGE ON HOT DUCTING.

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WE DEPARTED DTW ARPT WITH A NORMAL TKOF UNDER VMC CONDITIONS. AT APPROX 4500 FT MSL I RECEIVED A CALL FROM THE LEAD FLT ATTENDANT INFORMING ME THAT THE CABIN WAS VERY HOT AND THERE MAY BE SMOKE IN THE CABIN. I INFORMED THE CAPT OF THE LEAD FLT ATTENDANT'S CALL. BEFORE THE CAPT COULD RESPOND; THE INTERPHONE 'RANG' AGAIN AND THE FLT ATTENDANT RPTED THAT THERE WAS DEFINITELY SMOKE IN THE CABIN. EVEN BEFORE I COULD RPT THIS TO THE CAPT; HE SAID 'LET'S GO BACK.' THE LEAD FLT ATTENDANT KNOCKED AND ENTERED THE COCKPIT AND RPTED THAT IT WAS STILL VERY HOT AND THE SMOKE WAS WORSENING. I THEN CALLED DTW DEP AND STATED THAT WE NEEDED TO RETURN TO DTW. THIS WAS AN OVERWT LNDG WITH A WT OF +/-102500 LBS AND OUR MAX LNDG WT IS 99000 LBS. PER OUR FLC OPERATING MANUAL; WE ACCEPT THE OVERWT LNDG IF A GREATER EMER EXISTS. THE CAPT MADE A SMOOTH TOUCHDOWN AND ROLLOUT WITH MINIMUM BRAKING CONSISTENT WITH SAFETY (BOTH RWY LENGTH AND ACFT WT/BRAKE ENERGY). AFTER CONFIRMING THERE WAS NO FIRE ON BOARD; THE CAPT RADIOED FOR THE FIRE EQUIP TO STAND DOWN. THE CAPT TAXIED TO THE GATE AFTER MAKING AN APPROPRIATE PA ANNOUNCEMENT. I RAN THE APPROPRIATE CHKLISTS. AFTER ARRIVING AT THE GATE; WE DEPLANED PAX AND SPOKE WITH MAINT. IN A LATER CONVERSATION WITH MAINT; WE WERE TOLD THAT THERE WAS SOME OIL LEAKING FROM THE SEALS OF THE R PACK. MAINT FELT THAT WHEN THE R PACK OVERHEATED; SOME OF THE OIL WAS HEATED TO THE POINT THAT SMOKE WAS CREATED AND THAT SMOKE WAS THEN DRAWN INTO THE CABIN THROUGH THE AIR CONDITIONING SYS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE PROB WAS TRACED TO A LEAKING AIR CYCLE MACHINE ON THE R PACK WHICH ALLOWED LUBE OIL TO ENTER INTO AND ONTO THE HOT DUCTING. THE RPTR SAID TO THE RPTR'S KNOWLEDGE THE AIR CYCLE MACHINE WAS REPLACED.