SMOKE AND FUMES IN COCKPIT AND CABIN AFTER CHANGING FROM ENG TO APU BLEED DURING APCH. FOUND TO BE OIL IN DUCTING FROM APU OIL SEAL.

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DURING DSCNT INTO MYR; WE XFERRED THE PACKS FROM THE ENG BLEEDS TO THE APU BLEED. SHORTLY AFTER; WE HAD SMOKE IN THE COCKPIT. WE IMMEDIATELY DONNED OXYGEN MASK AND ESTABLISHED COM. THE FO WAS THE PF AND CONTINUED IN THAT ROLE. WE DECLARED AN EMER AND WERE THEN CLRED FOR THE STRAIGHT IN VISUAL APCH TO RWY 17. I THEN XFERRED THE PACKS BACK TO THE ENG BLEEDS. HOWEVER; WE STILL HAD SMOKE. BEING WE WERE BELOW 10000 FT AND ONLY 2 1/2 MINS TO LNDG; I THEN TURNED THE PACKS OFF AND THE SMOKE QUIT. I CALLED THE CABIN AND SHE CONFIRMED THERE WAS ALSO SMOKE IN THE CABIN. ON FINAL; THE SMOKE HAD CLRED FROM THE COCKPIT SO I REMOVED MY MASK. AFTER LNDG AND CLRING THE RWY; WE VISUALLY AND VERBALLY CONFIRMED THE CONDITION OF THE CABIN AND ELECTED TO CONTINUE TO THE GATE AS OPPOSED TO EVACING ON THE TXWY. ALL PAX APPEARED TO BE FINE AND NONE REQUESTED MEDICAL ATTN. THE NEXT DAY AFTER SPEAKING TO MAINT; THE CAUSE WAS DUE TO THE APU BLOWING A SEAL AND DUMPING OIL INTO THE BLEED SYS. WHEN WE XFERRED BACK TO THE ENG SYS THE OIL WAS STILL PRESET AND THIS WAS WHY THAT DID NOT HELP THE SMOKE INITIALLY. MAINT ALSO SAID WE WERE LUCKY BECAUSE THE SMOKE IS NORMALLY MUCH HEAVIER. I WILL SAY THE SPD AT WHICH WE XFERRED THE BLEEDS AND DISPOSED OF THE PACKS HELPED. DUE TO THE CLOSE PROX OF THE ARPT AND IMMINENT LNDG; THE QRH WAS NOT CONSULTED AND SYS KNOWLEDGE AND MANY YRS OF EXPERIENCE DROVE THE STEPS AND SAVED LOTS OF TIME AND PERHAPS MORE.