As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED SMOKE IN COCKPIT AFTER APPLYING CABIN HEAT. REMOVED COWLING AND INSPECTED MUFFLER ASSEMBLY. SMALL OIL STAIN ON MUFFLER SHROWD. FOUND OIL RETURN LINES WERE SLIGHTLY 'WET' WITH OIL. TIGHTENED OIL RETURN LINE CLAMPS. WASHED DOWN ENGINE AND MUFFLER SHROWD. MADE A LOG BOOK ENTRY, AND RETURNED TO SERVICE.