For synthetic, use Mobil Delvac-I for diesel or Delo Synthetic, both are CI-4+ rated which is far higher than gas specific Mobil-I regular.
As far as best oil goes, check out the various UOA in Bob Is The Oil Guy forum at
http://theoildrop.server101.com/cgi/ultimatebb.cgi
You will be pleasently surprised how well group-II HDEO diesel oil like Delo 400 does against synthetic diesel oils, also Mobil's own Delvac MX which is group I with additives too manages fantastic results. Unless facing extremely low temperatures, using a group II oil like Delo 400 or even Delvac Super/MX changed at intervals of 5000 miles makes good economic sense and it provides excellent protection as well.