Interesting! The new Coats and Clark polyester is a strange thread. When you put it in the bobbin and pull, you can feel a strong vibration. When I go to sew-off a machine, I can tell if it's got Coats and Clark in the bobbin. We can't get a balanced zig-zag stitch with it. I've found that customers don't generally know what a balanced stitch looks like, and don't really care too much. But it certainly doesn't damage the machine. The old Coats and Clark had cotton wrapped around a poly core. It has a better stitch quality, but is more linty. As long as you're regularly cleaning the bobbin area, it shouldn't be a problem. Poly thread generally needs a tighter bobbin tension than cotton thread.