I had to some research about oils several years ago and found that the modifiers that make multi-vis oil thinner in cold and "not so thin" in warm are pretty heavily engineered substances.
Apparently tests have showed that modern high tolerance engines would destroy older oil formulae, so that the newest oils are much more capable of staying "in viscoity range". That is the key. In other words it isn't so much that your engine "needs" a more viscous oil as it heats up, as that new oils behave differently. A modern 5W20 provides all the protection that the engine needs AND the lower viscosity means there is less lost power slinging around a thicker oil.
Put another way, it isn' t so much that 30 weight oil does a better job at 90 or 100 degrees, its the new additives allow the 5W20 oil to stay 5W2o REGARDLESS of conditions, while the older formula 5W20 would "breakdown" and go out of range (usually btw to a more viscous, harder to put non-uniform sludgy mess -- not as some people might think to a "watery" less viscous/less clingly/less protective disaster).
I think the Pennzoil site still has some of this info, though I haven't checked lately.