I did my first overnighter this weekend with Rancher...19 hrs of 230-260F (I increased at the end)...using my Stoker.
At the 15 hr mark, I asked for temp to go from 240 to 260 to help finish the two butts faster...there was NO problem.
I never opened the WSM the entire 19 hrs. In fact, I havent even checked how much Rancher is left..probably not too much but with Kingsford, I would have had to open the door or even stir it to get the temps up that late in the cook.
I love this stuff!