I have owned 3 of these npr with diesel engines. Any of the 3 big players (rotella, delo, and delvac) will b fine. I ran which ever one was cheaper.
As far as viscoity it the manuel don't show it (though its n there some where) look on the driver's sun viser. It's on the back side and it shows anything from 5w30 to 40wt and the temps that each is acceptiable. though 15w40 has almost as big of temp alowence as 10w30. You can run either with out problem.
These engines are basiclly 1/2 of a durmax engine. I ran mine hard and only did routine mantiance nothing more.
03 ccsb,2500hd 180k+ , 265/75/16, D/A, PURE POWER FILTERS, Bilstin Shocks,Aux Springs,Swiss Cheese Air box W/Fram Tough Guard,BF Goodwrench K/O