I have recently discovered the joys of bobbin thread for the thread in the bobbin - it blends into the back so nicely and more fits on the bobbin so you are not changing it as often. Sometimes what dictates what I use is the color I need. I do try to use a good low lint thread like Aurifil 50 wt thread. (I used a cotton threat from my LQS and there was so much lint I had to clean out the bobbin area every time I changed the bobbin.)