I don't think conventional oils resist breakdown with varnish and sludging, etc as well as fully synthetic oils.
There's a lot more than just wear rates to consider when comparing conventional and synthetic lubes.
I certainly agree. One should not extend out the OCIs without understand all the risks. Varnish is actually harmless. Sludge is a different matter, though.
In my two MGMs (4.6L Ford engines), I've run over 150k combined miles of "extended" OCIs. Typically 10k miles, but some longer. The under-valve-cover conditions have always looked fine, even after the long OCIs. In our departed 1995 Villager, I ran 10k and 15k mile OCIs; again no sludge under the valve covers. What my experiments show is that in those engines, there was no risk. I have since sold those two cars to my son; he continues the 10k mile OCIs.
However, there are engines that do not tolerate longer OCIs, because of engine design issues. For example, the Saturn SL2 engines have rings with no drain-back gaps; they tend to sludge the oil at the ring lands and cause stuck rings. Some of the Toyota 4-cyl engines ran very hot in the heads, and so they'd sludge up a conventional oil fairly quickly. Here, even following the OEM schedule didn't stop the issue from happening, so it's not like shorter OCIs solve all problems, any more than longer OCIs always will cause problems. Many engines can tolerate longer OCIs; some cannot, simply because of inherent design issues.
Additionally, the ability to control soot/insolubles is not only about the engine design, but the lube fortification, also. While it is true that PAO syns do resist oxidation better than typical dino oils, those same PAO lubes also do not hold additives in suspension well, and so group II or II+ lube is blended in, so that it holds the additives in suspension. Conventional oil base stocks (II, II+, III) hold additives in suspension very well. And so the anti-agglomerates and detergents can be quite robust in a dino lube product. Those, too, help keep engines clean. As the API service categories are indexed, the requirements to control soot/insolubles increases in both gas and diesel lubes. Lubes today are far more capable than lubes from years ago.
You have to know what engine you have (what strengths and weaknesses it has by design), what lube benefits and concerns exist, as well as the understanding of how to use multiple tools (UOAs, PCs, visual observations, etc) all in concert, to have a successful program. I have NEVER advocated for blind extensions of OCIs without using the tools at hand. But you have to know what those tools are and are not capable of, and then practice the right approach. The reality is that most all engines have wear rates drop out to 15k miles; that is beyond debate. After that, you have to make educated decisions based upon the type engine you have, it's known pedigree, and be willing to put effort into assuring equipment health with using all the available tools.
In my garage, I practice longer OCIs. But I don't do it blindly, and I don't tell others to do it either. What I tell them is if the UOAs is showing good wear rates, and the engine family is not known to have sludge issues, then it's OK to extend the OCI and experiment.
I recently sold my 1966 289ci 2-v Mustang. Before I did, I cleaned up the engine and replaced the valve cover gaskets, which had never been off in the 76k miles since new (we got it used as second owners). I was horrified to see the huge amount of sludge under the valve covers. The first 60k miles under the first owner were unknown to us; I have no idea what OCI they did. But lubes back in the 1960s and 1970s and even early 1980s simply were not fortified well. Hence, even "normal" engines running "normal" OCIs were prone to sludge.
There was a time when 3k miles was the normal OCI recommendation from many sources. Then it became 5k miles. Then 7.5k miles. Now, cars/trucks with IOLMs are showing 10k miles to be common. Even the OEMs are realizing that there's little risk to longer OCIs, as long as the engine design supports it. My 2006 Dmax IOLM would indicate 9-11k miles for an OCI. My brand new Taurus' 3.5L engines both indicate 10k miles + via the IOLM. Data shows that those engines are very easy on oil, hence extended OCIs are not a big risk.
Most engines in the last two decades have been designed well. Most all lubes in the last several API iterations have been fortified very well. Engines run very "clean" today. The risk of sludge is very remote despite all the fear mongering.
If someone is unwilling/unable to practice a full spectrum maintenance program, where UOAs, PCs and under-cover inspections take place, and don't understand the nature of their specific engine, well then I cannot be held accountable for their inept approach if they choose to only follow part of my advice, and not all of it.
But my comments as to wear rates is 100% solid; nearly all engines show lower wear as the OCI matures, at least out to 15k miles, where my data streams typically end.