I always use the same thread in the bobbin as on the top. I also change my needle with every quilt. I like Glide Thread and Permacore Thread. Both work well in my Gammill. I use a size 3.5 needle with the Glide and a 4.0 needle with the Permacore. I have never had a shredding problem. I check the bobbin tension with my Towa Gauge with every bobbin change. I check the top tension on a scrap quilt sandwich before I start a quilt. The most I usually need to correct on the top is 1/2 turn (either direction). I tried cotton thread when I first got my machine, but had a lot of breakage so I decided I did not need that aggravation. Trial and error is how I came up with what works for my machine, but maybe this can help you.