Looking at the Ask Mobil archive, I see a very interesting answer on the topic: "Phosphorous Levels of Mobil Conventional Oils"
In response to a guy with a classic car who is concerned about zddp levels and only drives 1000 miles between oil changes, Mobil says
... The good news is that if you are only driving 1000 miles between oil changes, the 800 ppm phosphorus will provide adequate protection in the older vehicle at this short oil change interval.
I own a good amount of classic cars and the base level of ZDDP has always been around 1000 PPM or higher. The HM syns that Mobil 1 has are around 1000 and in my opinion are very good oils if you want a syn motor oil.