And this is entirely determined by accountants and lawyers, not mechanics and engineers. Manufacturers tell you to do many things that are not in your best interest. What about all those sealed automatic transmissions they have now? Absurd.
The accountants certainly play a role in the single grade requirement (CAFE), but engineers most definitely play a significant role in determining the tests and their parameters that define the approvals.
If you ran an engine that "requires" synthetic with regular oil and then changed back again nobody could possibly ever see the difference. We can all agree it is better, but exactly how much, when, and where, is nearly impossible to quantify in any meaningful way.
It's easy to qualify, that's why the controlled tests exist. Porsche A40 and C30 for example, part of the approval is simulated lapping of the Nurburgring race track for an extended duration. This is followed by a tear-down with measurements. Ergo, it qualifies the oil's performance under those conditions.
As these approvals have become more and more strict, like even Dexos, for the non-Euro stuff, conventional oils have simply been unable to meet the performance requirements. They can't pass the oxidation limits, they can't provide low enough volatility...etc. While there have been some base oil breakthroughs, like XOM's EHC Group II+ bases, ultimately the deficiencies with conventional bases are easy to qualify.
"For life" transmission fluid is more about the optics of low maintenance costs to attract buyers. We aren't seeing that with oils. What we are seeing is engines that are becoming more and more complex and manufacturers simultaneously trying to get better and better fuel economy out of them.
This is why it's been an internet argument as long as there has been internet. You would have to dyno test thousands of samples in controlled conditions that even the manufacturer can barely simulate.
But that's exactly what even the basic API approval sequences do. For the more stringent approvals, additional tests are added, limits are made more strict...etc. Even then, sometimes the API approvals just aren't enough with certain engine designs that are prone to creating issues, such as the Toyota engines with inadequate oil drainback holes in the oil control ring lands and Honda's VCM V6, which tends to both stick the rings as well as carbon/varnish up the one head.
This is becoming more and more important now that everything is DI with a turbo on it and much harder on oil than the port-injected naturally aspirated mills of the past with far lower power density.
They have to write things like that in manuals because some moron will buy a new Toyota and try to put some straight 30 from Walmart in there.
That's true. But they also have to stipulate people used an approved lubricant because somebody with an M3 might put 5W-30 conventional in it and then go hooning and pop a rod through the side of the block.
"Back in the day" there was a sludge epidemic with VW's because dealers were using "regular" Castrol Syntec 5W-30, not the Euro extended drain 5W-30 that was stipulated in the manual with the VW approval. There's a difference, one that becomes very apparent when you are running the manufacturer-mandated drain intervals.
Anyways, to bring this back to UCL's, it would be quite easy for their performance to be qualified via standardized testing. This is not being done could be as simple as nobody as bothered, but it may also suggest that somebody did test and the results weren't going to be great advertising (IE, it did nothing, or next to nothing).