I wonder how many Lube-savy BITOG viewers continue to opt for 20 wt. oil when its reasonably clear a 30 wt. will protect better. I say that bc the science of lubrication hasn't changed much in the last 70+years. (Not saying lub products have not gotten better). We are talking theory. And I am not faulting anyone that choses to use 30 wt or not use 30 wt.
Before manufactures went with 20 wt. oils I believe "the requirement" was that the vehicle engine would last 200K miles. It was determined that 150K miles was dooable.
The question is "why" do users go with 30 wt. Is it "economy" or warranty? Or something else.
I used the 20 wt. for my '19 Crosstrek and switched to 30 wt when I thought I would be keeping for a while (I didn't). I switched to 30 wt after 1000 miles on my '22Forester. And note the 20wt. oil can not adequately protect the Subaru FA Turbo engine...but it can protect the FA unturboed engine. Again not defending my actions. Strictly curiosity.
From the articles I've read, it's the 2.6cP HTHS that seems to define the floor for where you have to do "other things" to gain long term durability, such as utilize special coatings, implement wider bearings...etc. This is due to increased operation in mixed and boundary regimes vs hydrodynamic, and with components that historically had only operated in hydrodynamic. This is where many of the breakthroughs in additive technology have also had their focus, to deal with operation in mixed/boundary, but heavier oils like the xW-20+ grades have benefited from this science as well.
You have to remember that historically, oils were far inferior to those developed today in terms of stability, staying in grade, and the like, particularly when we are discussing the oils that dominated the oil change scene, which were conventional. Oils like Mobil 1, using PAO, were vast outliers and didn't represent the standard of the day.
A 5w-30 during the 70's, 80's and 90's, would have used primitive VII polymers that would rapidly shear, and cheap, light, and highly volatile base oils. It's quite likely that the engines of the day were actually running on an xW-20, and possibly one near the bottom of the range, despite the 5w-30 on the bottle. 10w-30's would have been a bit better, using slightly heavier bases.
The implementation of more stringent manufacturer and even API approvals in the last two decades for the North American marques, which aligns them more closely with the European OEM's, who started doing that in the 90's, has led to a dramatic increase in the quality of lubricants as well as pushing the oil companies to improve additives. This is also why it's very difficult to find a true conventional oil now, because they just lack the ability to meet the current performance requirements.
An oil like M1 EP 0w-20, using a personal example here, being ~70% PAO, is going to stay in grade and will be formulated with cutting edge AW and AF additive chemistry when compared to say SJ or SL vintage Castrol GTX, PYB, or Valvoline white bottle in the 5w-30 or 10w-30 grade, which will use light Group II and even maybe some Group I base stocks and a big shot of cheap VII.
Now of course modern xW-30's have improved too, benefiting from these same developments, but the above is more in reference to the perceived reduction in intended equipment operating life. I think you'll find that these engines that spec'd a conventional xW-30 were, in reality, spending most of the interval on a functional xW-20 due to VII breakdown. Shear resistance didn't even really become a thing until the Euros started rolling it into ACEA protocols and the Long Life oils started to appear on the scene.