I use mostly poly. I like Permacore, Glide and YLI variegated. I have tried So Fine and it's okay. A friend likes Omni and another friend uses mostly Signature Cotton both of which I have yet to try. We all have different models of Gammills. I have the Bobbin Tension Gauge that I use with every bobbin change and I swear by this. I have had my machine a little over a year and have never had any tension problems. (Knock on wood.)