I think we've been over this a gazillion times. If you take a HDEO, and change it at 3k miles, then ZDDP and other components become significant gas path contaminants.
If you change a HDEO at a more reasonable interval (ie: 10k+ miles -- very well within the capabilities of XD-3 synth), then the exposure is substantially reduced.
The EPA's efforts to have ZDDP levels reduced in motor oils are mostly due to the propensity of the American motorist to change their oil every 3-5k miles, instead of at, or exceeding their manufacturers' recommendations in terms of mileage. If everyone would change their (synthetic) oil according to the results of UOA, there is no doubt in my mind that ZDDP contamination would not be an issue.