My guess would be you can/should use Rotella T 15W40 or T6 5W40. The T6 is synthetic and more expensive. Both are excellent oils and are used in many diesel trucks, tractors.
15w-40 Diesel oil or you can also use 10w-30 Rotella in cold weather. This is what my 2002 GMC Savana 3500 6.5L Turbo Diesel owners manual used to call for. HTH
Just about any name brand oil that is CJ-4 should work fine. Mystik makes a pretty good 15w40 synblend that can be had at farm and home stores at a pretty good price. Haven't looked lately, but got some of it late last summer for $11 a gallon. My Cummins really liked the stuff.