OVERKILL
$100 Site Donor 2021
I've pointed this out for years, so my apologies for stating it once again, but the problem with used oil analysis used recreationally is the desire to read more into the results than they can convey. This presents because they are the only tool perceived as providing this wear "insight".You do have a very good point. While I strongly believe "garbage in garbage out" principle, and would not want to rely on UOA for wear results, a corpus of thousands of UOA can provide some value there, even if they are not a meaningful metric for a single engine.
I also have been saying that higher viscosity does not necessarily mean better protection. If that was the case, additives would not be the most expensive part of an oil. (Though a caveat here, insufficient HTHS is definitely a source of wear. So for high temp applications, going higher in viscosity, which in turn a proxy for hths, is crucial. Does not metter if that is for the engine, or gearbox/diff)
It's the old "when you've only got a hammer, everything looks like a nail". used oil analysis are the only cost-effective tool we have available, so of course the desire to be able to divine wear performance of individual lubricants and use this data comparatively will naturally emerge, but this necessarily involves acknowledging that you are ignoring the limitations of this tool. It's like trying to run precision rifle with a shotgun.
Blackstone, who of course has a massive UOA database, has stated that there is no observable difference in wear performance between spec lubricants when contrasting used oil analysis. This does not mean wear performance differences don't exist, but that you aren't able to flesh-out what those differences are using this tool, which shouldn't be surprising, given that this is not one of the purposes of the tool.
And you are quite right, viscosity alone is not some singular determinant of wear prevention and control. It's part of a system that involves the additive package, whose role is to prevent and reduce wear in areas that are not protected exclusively via viscosity, such as components functioning in the mixed, mixed/boundary or boundary lubrication regimes.
That said, the lower you go with viscosity, the more areas that potentially move left on the Stribeck curve, moving out of no wear (hydrodynamic) into mixed, mixed/boundary or even boundary, where some manner of wear, no matter how small, is occurring. This is why I brought up the 0W-16/0W-12/0W-8 testing and conversation we had on here years ago earlier in the thread, because it was all about acceptable wear as a result of reducing friction and improving fuel economy.
This is also why I brought up the API/ILSAC carve-outs for the thinner oils, where the bar has been lowered on a few of the tests/parameters. But, as I noted then, this does not have to be the case, Mercedes for example, does not adjust their wear performance limits for thinner oils in the same engine test. This doesn't mean that a full-SAPS oil might not actually perform better, but at least the bar is held constant regardless of viscosity.