Running or "shooting" lines are most commonly used with Skagit style line setups. They are also necessary for most Scandi style shooting heads. The running line is mean to be thin and be towed or shot out by a heavy shooting head. Mono running lines shoot the best, and thinner is better for distance but there is a balance. Thicker is easier to handle. On trout spey setups we like a 30-35# running line, switch rods for summer steelhead 40#, and any full spey setup should have 50# mono. If you prefer a running line that is easier to handle in the cold with gloves on, consider one of the traditional running lines that are not mono.