15w40 is good down to 30 degrees for regular duty, and I think down to 10 degrees for towing. Plowing seems similar to towing, so maybe 15w40 is the way to go? If it's colder than 10 degrees where you are and you want something thinner you could always go with the new 5w40 diesel oil that the 6.4 uses. It's compatible with the 6.0's and 7.3's.