There's more to "zinc" than just boundary protection or cam protection. It is used also as an anti-oxidant, among others, for protection of oil from thermal breakdown. I don't think it will ever go away completely and most good oils have it already in their additive package. That's mainly why I do not condone doping good oil with off the shelf additives. The chemistry can get messed up.
As to viscosity, we really need to focus on what's printed in the Owner's Manual. Sure the oil has changed, but the engines have morphed as well. As air cooled goes, these engines used to have looser fits and why the OEMs often pointed out in their lube tables that going anything thinner than the 1st choice may increase oil consumption such as going 10-30 instead of straight 30 for operating temps between 32* and 100* outside.
Checking build specs, the K series for example, there were 3 styles of pistons. Style A had a .007"-.010" cylinder clearance, C&D styles had .003"-.005". Even water cooled truck engines would have piston diameters over 4" before they get that close to that clearance while ours are barely 3 to 3 3/4". That's a lot of slopp'in piston rock'in go'in on at start up until things warm up and expand. I would want some oil body to cushion the gap. Now granted as things get colder outside the viscosity requirement drops, but it's all relative and there will still be "thickness".
For comparison the Kohler 7500 series manuals recommend 10-30, but their piston fits are only .0007-.002" clearance. That's right, 3 zeros is not a typo.
The Command Pro commercial series recommends 10-30 or 20-50 oil depending on service and ambient temps. 2 styles of pistons are listed: A with a .0015-.003" fit and a B with .0007" - .002".
The Kohler website does not list which oil to use, instead they simply tell you to find it in the Owner's Manual, there are just to many variables, engines, and OEM requirements so that is where truth is, the Owner's Manual.
So just with an overview I can see older engines with looser fits need thicker if not also straight weight oils and adjusted to conditions such as outside temps.
Newer engine designs have tighter fits and work well with more modern oils particularly multi-viscosity and synthetic blends.