"The thing you need to do is check this about once every six months."
Truer words were never spoken. I worked years ago with all the major oil companies (Amoco, Exxon, Mobil, all the PA based oil refiners like Penzoil, Quaker State, Wolf's Head, etc.) as we helped them market tank truck loads of oil. Also local re-refiners like Motor Oils Refinery where used oil was repurposed.
I learned that it's really most often the local or regional blender/distributor/reseller who decides what additive package goes into your bottle of motor oil. And they can and do often change the mix or percentage of ZDDP or any other compound at the drop of a hat. BE SURE that your purchased oil has a current tech sheet that matches the rating on the bottle's top and/or is current within the year of the tech sheet.
It would be really cool to develop say a ZDDP test strip that actually shows the percentage of said compound in a particular bottle. That ain't gonna happen though!