A 50/50 antifreeze/water mix protects to -37 degrees F. A 70/30 antifreeze/water mix goes to -69 degrees. Straight ethylene glycol without water in it freezes at +9 degrees, so a 70/30 mix is the strongest solution that's reccomended. Block and/or oil dipstick heaters just help the engine start by keeping the engine slightly warmer than the outside temperature. Diesel engines with 15W40 oil run it year-round without oil-related starting problems, so there's no reason to use a lighter weight oil. The original winter oil that was reccomended for your engine was 20W-20, so the 15 weight cold specification has better flow during cold start conditions than when the engine was new.
Jerry
