I use any size that has the color I want. The thread type doesn't matter that much as long as it's quality thread. The needle size is what is important to sew with few thread problems. Most of the time I use the same wt in the bobbin but maybe a different color. Don't get caught up in the 'right' thread to use. Remember all the hype for this thread or that comes from the ones that profit someway to promote the brand.