On our offset, I first wiped the inside, then coated it all over inside the cooking chamber with cooking oil. Having gone to a butcher and gotten several pounds of pork and beef fat trimmings, these were placed on the food racks. The smoker was then run at around 300-325°F for about four hours: smelled great and gave a real sheen to the inside.
I wiped everything down when I initially assembled my WSM.
I skipped the cooking oil and smoked several pounds of brisket fat @ 225. Rinse and repeated. It left a nice coating/sheen that helped seal up my WSM.