My Viking is programmed for 40 weight thread - so as long as I use a 40 weight, all will be good. According to the manual for the machine I can change the tension for the different weight of thread, but my repairman said that works well with sewing but not the embroidery. He said I can do it, but don't be surprised if I don't like the results. So I haven't bothered to try using a different weight.