I have had good luck with LN Engineering, they sell Brad Penn and they have a decent write up on oils on their website (too bad it is for Porsche).
http://www.lnengineering.com/oil.html
The article has some decent technical content on ZDDP and about using "diesel" oils. He addresses adding too much ZDDP as well and what these additive can do for you. It is a good technical article. I would recommend calling him if you have questions, he is very pleasant and has helped me out in the past, even though I have never owned a Porsche!