There has been much discussion on the web about the fact that the latest oil formulations have reduced, or eliminated, their ZDDT and Phosphorus content. There are still specialty oils available which have ZDDT levels at 1200ppm or above. However modern oils, even our beloved Castrol 20W50, fall far below this mark.
Frank Mount advises me that he uses one can (500ml) of AC Delco Engine Oil Supplement with each oil change. AC Delco EOS contains 6221ppm of ZDDT and 5762ppm of Phosphorus. It is readily available at GM dealerships and retails for $8.20. On November 20, 2010 I bought 3 cans at City Buick and was given a volume discount to $6.50 per can!
I, for what that's worth, am following Frank's advice.
AC Delco Engine Oil Supplement is available at GM dealers as Part # 992869
For a treatise on how ZDDP works, (and what ZDDP stands for!) I suggest that you look at http://www.sherbournemews.com/oil.pdf.
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Last revision: 04-Dec-2010