Viscosity index is really the ability of an oil to maintain its thickness or thinness(viscosity) over a WIDE temperature range.
It has nothing to do with lubricating abilities. The index is more usefull for temperature climates where huge temp swings are normal year round, say 20 deg F to 90 F year round. The higher the index the better for these locations.

At 212 F(100 Centigrade) engine temps 5W30 10W30 Straight 30
oils have the same viscosity, they are the same oil. PAg malamig lang nagkakatalo! Ngayon, pag 30 degress F na ang weather dyan matatalo ang straight weight oil. Like I said in earlier post
when do we get 30 degree weather in RP?

Kaya ko lang gusto ang straight weight sa hotter running engines like turbodiesels is that at higher viscosities (straight 40 or 50 it withstands heat a lot better and wala syang halo na viscosity improvers which tend to break down in high heat.

BTW Australia is known to be a hot place, your right, but they use multigrades there because it also gets real COLD in a lot of places BRRRRRR