As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CABIN FILLED WITH SMOKE ACCOMPANIED BY STRONG OIL SMELL AFTER TAKEOFF. FLIGHT RETURNED TO DTW AND AIRCRAFT LANDED WITHOUT INCIDENT. FOUND OIL RESIDUE IN PACK DUCTING FROM PREVIOUS APU PROBLEM. CLEANED DUCTING, REPLACED WATER SEPARATOR BAGS AND OPERATIONAL CHECK OK.