Deerefan: if you want a synthetic it would be Delvac 1 and 5-40-Mobil Turbo Diesel Truck Oil is Del 1, this confirmed to me by Mobil.
This is what I have found out in our E450 6.0, '07: I switched from whatever the factory fill is to full synthetic when we took delivery of the rv from the upfitter, about 700 miles.
My psd, which I love-37k miles of flawless operation- 14/16 mpg (remember it is an rv, 14k-gvw), in fact it is the only good thing about the crappy rv we bought really likes to have 15-40 oil. To date I have tried two fills of Del 1, three fills of Amsoil Marine Heavy duty diesel, one fill of Amsoil 30 10/30 diesel oil, and one fill of full syn Rotella-I change roughly every 4k miles.
Whenever I have switched from 15-40 to another weight I see very very slight oil consumption, less than a quart-with the exception of Amsoil 30 weight which was even less -with the 15-40 the level on the stick is the same at change and fill.
I am going back to 15-40 and staying there, and I am going back to Amsoil permanently also, there really is a difference and I DO NOT sell the stuff.