The original recommendation found in the 2003 service
manual was SAE 20wt. for above 50 degrees F, and SAE 10 for below 50degrees F. The oil quality was CD.
As most of today’s diesel
oil ratings are backward compatible to the CD spec, I would suggest a quality multi viscosity diesel
oil such as Shell Rotella T, 15w/40. This oil will carry a rating of something like CI or CJ, so will meet the quality requirements that you need, plus will provide excellent service
regardless of most any ambient temperature you are likely to use the engine in.
Other manufacturers such as Exxon/Mobile, Total, etc. also make excellent diesel oils that can be used as well.