My memory is the VI is another way of describing the spread between 40 C and 100 C. But I'm going to have to reread the article to be sure.

Since this is the 'race' forum, the important temperature ought to be done with a temperature sensor in the oil pan. So, for example, my old 340 ran 280-300 F on the highway. There were some issues in my exhaust and remote oil filter routing, which I eventually addressed. It still ran 260-270F at highway speeds. Unfortunately, I didn't understand about pour points and was suspicous of the synthetic sales pitches. So I ran non synthetic 20W-50 racing oil. That might have made sense for the highway driving and racing once warmed up, but probably was not getting the flow needed until it was up to full temperature. Certainly was caused accusumps 100 psi relief valve to pop open a few times. yuck.

The oil in my current 340 runs 160-190 degrees F on the street. At the beginning of a run, sometimes its not even at 160. So for this engine, both the 40 C and 100 C viscosity are important to me, as is the pour point. With the higher valve spring pressures, the tried and true ZDDP type anti-wear package was also important. Brad Penn 5W-30 fit the bill.
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edit: That was a long way of saying that I've come around to placing first priority on viscosity.
The article that helped me understand that was Oil 101 by AE Haas. There's a couple versions floating around the web. I linked this one because it has the final paragraphs where he starts to get into temperature on road course versus street driving. Haas was a physician applying his understanding of fluids to explain viscosity, pressure and flow to his fellow auto enthusiasts.