Soooo, the +/- iron content PPM isn’t an indicator of more or less metal on metal wear?
But I’m not purposely drawing any unwarranted conclusions. I’m going off what I’ve read a lot from from other forums but according to you that’s inaccurate so maybe it is faulty understanding.
The iron content measured in a UOA does correlate pretty well with wear rates. The issue is that oil formulation normally has a very small effect on these wear rates, whereas engine operating conditions have a large effect on wear rates.
For instance, iron wear from piston rings and cylinder liners is highly dependent on engine operating conditions, mainly sensitive to high engine loads and cold starts. These components aren't very sensitive to anti-wear additives. In fact, there are no tests in the API standards that even measure this wear.
On the other hand, valvetrain wear is very consistent. These components can be sensitive to anti-wear and FM additives, but for most engines, any oil that is fully formulated to meet a certain standard will perform very similarly. Some engines, like the one chosen for the valvetrain wear test used by API, are sensitive to ZDDP and moly content. However, even for these engines, there may not be a large difference between an oil with 600 ppm phosphorus and one with 800 ppm.
So as an example, an engine may end up with 5 to 6 ppm baseline iron from the valvetrain and timing system, another 1 to 10 ppm from the rings and liners based on engine operating conditions, and maybe +- 1 ppm based on oil formulation. Any effect the oil formulation has gets lost in the noise.
This doesn't mean that some oils are not a lot better than others, it's just that it's not normal wear rates that set them apart.