Part of what they do in oil testing is to test the oil's viscosity at several different temperatures, then as the last step a sample of the oil goes into a mass spectrometer for a molecular element analysis.
Yes, it is sold as a ZDDP additive, which is a
complex compound, but it is reported on the report sheets as a % of the basic elements that compose the oil additive ZDDP.
And yes, years ago there was an off brand of cheap oil that just put some phosphorus and zinc powders into their oil as their "additive package" and then when it came up with a really whacky low VOA they told the customer "to shake the oil up some" and retest it.
Needless to say Petroleum Institute of America soon hooted that supplier out of existence after their subsequent series of tests on that supplier's "oil" which turned out to be filtered recycled oil with "additives replenished".
However, I am still amazed no one has posted a VOA of Rotella T-4 after 2 months of sales .....