I have used Gutermann 100% cotton in my Janome with no problems, I also use Connecting Threads- Essential thread in the machine too - no problems other than more lint which can be solved by using a q-tip and swipe it out before you load your next bobbin - also Coats and Clark. Sometimes a dealer might recommend one thread over another because it is something they personally had trouble with in the past or because they sell something else - my opinion.