Dodge recommends a 5w40 synthetic below -18. I run Mobil delvac 0w40 year round without issue and the cost isn't much more than some regular 15w40 oils. My truck starts in the coldest of weather with ease. Take a 15w40 Dino oil and a 0w40 synthetic at -30 and see how they pour. Then think about all metal to metal wear when you first fire it up with 15w40.
15w40 is only good for warmer months as far as I'm concerned.