I use 50 weight for piecing. Recently used Superior thread variegated king Tut(40wt) in bottom as I wanted it to be a visible design on the backing and. I used a Bottom Line (60wt) for the top. Combo worked fine and I know people often do the reverse. I have not used 30 wt. for MQ but I agree with the others. I'm sure it would work fine if you adjust the tension and have correct needle.