Do they specifically call for conventional 15W40, or just 15W40 meeting service grade xxx or something along those lines? I've never heard of a modern engine explicitly specifying conventional motor oil. (5W40 is presumably synthetic, or at least synthetic blend, mainly because conventional oil of that viscosity isn't available.)
I personally would probably use 15W40 synthetic, Mobile 1 or whatever. It's not that much more costly at e.g. Wal-Mart, and spread over 15,000 miles any cost difference is immaterial. That's basically what I do with my car and my Ford-based motorhome--though neither are diesel, and the viscosities are different (as specified by the manufacturers).
