Some curves from comparing field data with lab ageing of a PCMO: https://acta.sze.hu/index.php/acta/article/view/546/515
Great paper, I’ve only just skimmed it, but I will read it in more detail soon.
While it does say “ZDDP depletes rapidly during the utilization of the engine oil” it then goes on to define this as “between 8,000 and 9,500km” (or about 5000 to 5900 miles) which is a normal OCI to me. This was for an oil that carries ACEA C3, API SN, BMW LL-04, MB 229.51/52, Dexos2 and VW 502/505. So a good quality but Low-SAPS oil, with TBN of 6.5 and Phos = 720 ppm with Zn = 880 ppm.
But I agree, we can’t read too much into the Phos levels, as there are many other antioxidants (AO) involved with this C3 oil, and probably with most other oils too. Plus I also agree that a synthetic oil, such as this C3 or the Rimula R6 will perform better, as their base oils are naturally more resistant to oxidation and breakdown.
However, the original test of the two 15W40 HDEOs, was an apples with apples comparison. Both were modern products being sold today, both were Group II based, both had a similar starting TBN and both had the same CK-4 rating. So while the Phos level is a bit of a leap of faith, on my assumption that they both had a fairly traditional add pack (without too much new secret sauce in either), it’s still true that one oil outlasted the other oil. I would assume this difference was due to the add pack, rather than any significant difference between the various Group II base stock.
Yes, Group III / Group IV should perform better, but my argument is once that add pack depletes, then the oil follows. It’s just that higher groups follow slower.