If you want to spend a little more than Rotella or Delo, this is what I use in my Mercedes diesel:



It seems to like it better than any other oil, starts faster in the cold than Delo ever did in that car and it seems to run a little smoother as well. Lots of 80's and 90's diesel Benz guys love it, and IIRC it carries a API CH rating (I have a bottle at home in my garage, I should check), so it probably has more of the good stuff our oils no longer contain.