Schaeffer's Supreme 7000 has a great additive package, you can find a ton of threads on bobistheoilguy.com about it. It's highly recommended for the Powerstroke trucks becuase it has a great amount of zinc, among other things. It's a synthetic blend oil and they do offer it in 15w-40 diesel-rated, but it can be tricky to find locally depending on your location.