Two things come into play for US destined vehicles.
1. A viscosity that'll take the engine through the warranty period and not significantly impact the overall longevity
2. A viscosity that’ll allow their CAFE (corporate average fleet economy) numbers to be sufficient to reduce their amount of tax credits.
For example, say a 5.3 Silverado gets .4 MPG more on 0W-20 vs a 5W-30, that tiny increase can greatly help their fleets corporate average and equal out to less money GM is paying out to meet US EPA regulations.
That same 5.3 in the Middle East may call for 5W-30 or even higher. I’m paraphrasing here, but I recall my K24 Accord used to call for 5W-40 for the same engine in Australia.
I don’t think engineers explicitly design, an engine around a certain grade, more so designed for a range. Just as an engine builder will tell you what grade of oil to run based upon bearing clearances, the same thing applies.
You get into the whole debate of certain 30 weights that are as thick as a 40 weight, then some 40 weights that are just a thick 30… the lines get really blurred.
Unless super extreme climates, or a mechanical failure, an engine just about anywhere in the world should be able to regulate itself where its operating temperature does not fluctuate. Of course, you may have extremes, where an engine is overheating, or an extreme cold start. However, those are not normal running conditions. With that said the same operating viscosity or up to temp viscosity should be just fine.