So, what was the issue with the lifters? The oil apparently didn't cause the issue, the lifters did. Replaced 'em and it ran. I would even bet you that she could run on 20W-50 just fine now that the lifter issue has been corrected.
And what's debatable about the answer I gave? Tighter tolerances during manufacture don't turn into oil passages that are so small that thinner oil is needed. If 10W-40 is too thick to flow through the oiling system then I'd contend that 5W-20 is not much better. What those manufacturing improvements do is get you an engine that runs better for more miles and with less wear to break it in.
Case in point: How viscous is any oil at 20°F? Fossil-based oil has parafin in it that needs to warm up to flow; synthetic flows much more easily when cold. If most engine wear happens while driving down the road then that which happens at start-up is of little consequence. But since most of it happens during cold starts you would naturally want to protect your engine the best way you can by using synthetic or blended oils to assure the oil will flow as quickly as possible at that time.
For anyone who wants to learn more (like I did), a trip to Bob Is The Oil Guy
with some reading is very eye-opening! I'm amazed at what I used to think was accurate until I went there and learned differently. Sure, some might say it's like drinking Kool-Aid, but ask them what they're drinking. (And maybe if they'll share some!
I just have a measure of difficulty accepting your contention that today's tighter tolerances require thinner oil. And your example has too many other variables in it that are undefined to make it a plausible reason to buy into your statement.