Oil manufacturers have no interest in selling you or recommending the wrong oil. That's one reason why they formulate so many grades. What they would prefer that you stick with their branding, and choose the proper oil from within their brand. So, a 15W50 oil has certain recommended uses in automotive, as does a 20W50, or 5W30.
In my old "foreign cars" of yesteryear, old Austins and MGs, etc. we always used Castrol 10W40, until they made a -50 weight. The reason we chose that, perhaps as just shade tree mechanics, is that these old engines were being utilized at much higher RPMs than they were for the English country roads for which they were initially designed.
Fast forward to today, and my current daily driver is just lumbering along at just over 2,000 RPM, with an extremely efficient cooling system, compared to my Pagoda. The same 75-80MPH highway speed is now at 4,000 RPM and running with a less efficient cooling system. Just a WAG here, but I would suspect that the engine oil requirements (less RPM, lower temp vs higher RPM, higher temp) would be different. On my many long trips in the Pagoda from Michigan to PUB, my car was doing 3800-4200 RPM for hours on end, frequently in the hot ambient temperatures of summer. The last long trip was Charlottesville to Michigan, a 600 mile, 9 hour drive done all at these speeds. I suspect that the 15W50 was a better oil to use than a 5W30.
Engine technology is wildly better today than 50 years ago. My Pagoda does 180HP stated with 2.8L, or 64.3 HP/L. My daily driver has 82 HP/L, and gets nearly 50% better fuel economy while unladen, weighs 62% more. Nothing special about my driver; it's just more modern, but hardly state of the art.
Oil has drastically changed too. The SAE specs SA, SB, SC, SD, SE, SG, SH are all obsolete, being obsoleted by engine technology. Current are SJ-SN Plus. Thus, utilizing any older recommendations from printed documentation of "the era" (say 1963-1971) doesn't take into account the oils of today, just the oils of the era--for which the specs are all obsolete.
One can comment all they want about not needing so much zinc and or phosphorus, but today's oils don't have them because first, they don't need them for modern engines and second, they will "poison" a catalytic converter.
So, when any oil manufacturer--I don't care if it's Castrol, Mobil1, Valvoline, Penn, Amsoil, or whatever--recommends a specific oil for older engines, I'm going to take their recommendation.
The best oil is one frequently changed!