Commercial running line most fly line manufacturers make in size 0.035 to no more than 0.040 (the numbers are marked on the spools they sell) will test at least 30 pounds. I find that such lines flow smoothly through the guides, rarely tangle and are thick enough to hold on to while fighting a large fish. Braided running lines are too rough on my fingers. Monofilament will let you sink your fly deeper, but for me even the large sizes are too thin. One thing you don't want is a weighted running line. It does not shoot well, and the head has to pull this heavier line to the target.