Originally Posted by SuperiorThreads

We have a library of education on our website that will help troubleshoot any tension problems that may arise as well as several reference guides to help choose which type of thread to use for your project!
http://www.superiorthreads.com/education/
http://www.superiorthreads.com/page/330/
It is absolutely fine to use a different weight and fiber for your top/bottom threads. As long as the thickness of both top and bottom are relatively close (A 40 wt. on top with a 50 wt on bottom), you can achieve a tight stitch. When you are using a thick thread on top with a very thin thread on bottom, stitches tend to become uneven.