BUt also often I am suspicious of that "Clearances" narrative because many engines that have been desigend decades ago also see these reduced thick oil recommendations ... so feel that CAFE fuel economy testing which is biased to short trip (where thinner oils do save fuel) plays a large role.
Also these engines oils are often recommended differently abroad, where CAFE does not apply.
For example my Nissan dealer in germany always recommended 10w-40 or 5w-40, for my Nissan 4.0 liter V6.
Its simply not possible for a 5w-20 or a 0w-20 to get good HighTempHighShear numbers even if its a synthetic..
Now fuel is expensive and of course I confess even on 0w-20 oil my engines would probably outlast me... but still.
