The lady who sold me my machine said to use polyester--not cotton-
I have never heard of any machine dealer say only use poly! That's very odd for a dealer to state her preference as what you should do. If the machine could only sew with poly then the dealer should have written facts why the manufacturer states only poly could be used. Some poly threads will score tension discs much faster than cotton. Sadly machines today, even the higher end ones, are not made to last. I test sewed on the newest Bernina, in the cabinet made for it and it vibrated horribly. To me constant vibration will mean something will break loose eventually. The dealer said it was mounted perfectly and that was just the way it sewed. It's a grand machine but I'll have to see one after it's been used for five years.