Oil grade was traditionally selected based on bottom end design (bearing width, clearances) and anticipated ambient conditions. This is why we used to have grade charts in manuals. As oils improved and we got wider spreads, the list was honed down until we arrive at a single recommendation for North America for all anticipated operating conditions. With HTHS limits of 2.6cP and higher, bearing clearances have been pretty consistent over many decades, which is why engines have been able to be back-spec'd to lighter oils (5W-20 for Ford for example) as long as oil temperature is suitably controlled.
So, if we take the Coyote, it was originally spec'd 5W-20. However, in order to ensure sufficient MOFT, engine power was thermally limited. That is, oil temperature was monitored and if it exceeded the acceptable threshold, available power would be reduced until oil temperature returned to within the acceptable limits. This is why Ford introduced the "Track Pack" version of the Mustang where the same engine, with the same clearances, but wearing a big oil cooler, was spec'd 5W-50.
The Coyote spec visc was later updated to 5W-30 is my understanding, which I assume was due to observed wear performance that was not deemed acceptable by Ford in service.
So grade may be a parameter in design (we are going to design this to see adequate wear performance on 0W-20) but until it's actually manufactured and tested, that remains a potential variable, and may still persist as a variable if post-production feedback from general service dictates. And of course anticipated operating profile also plays a role, that's why the 6.4L HEMI spec's 0W-40 while the 5.7L spec's 0W-20.
However, when you step below the 2.6cP HTHS realm into "ultra thin" territory, things change. All of a sudden engine design must cater to viscosity. Wider bearings are needed for example, along with special coatings on surfaces, and again, this is about "acceptable" wear, as you will have more components operating in mixed/boundary regimes the thinner the lube is, which means you are controlling wear, using additives, rather than avoiding it (hydrodynamic).