I use pretty much whatever I have on hand but I generally like cotton better. I guess I've just gotten into the habit of cleaning around my bobbin area and under the plate on a regular basis. With the machines I grew up with, it was necessary to oil the bobbin race every so often so I would just give the rest of the area a good cleaning while I was at it. I've gone almost completely over to Aurifil gray for piecing. It blends into every color. I do have a couple of scrap projects that I'm using up all my leftover threads on though.