ez,
The tests done measure viscosity at 40 and 100 deg C so they show what happens within an engine. Think that the performance of an oil at normal working temps of an engine matter more than ambient temps. A turbo adds stress on the oil, just as high sulfur fuel, egr's, soot. Used oil specs would give you an idea if the oil can handle all these factors.
If the oil were to breakdown then you form a sludge which can ruin an engine. This sludge can be prevented by the use of the right additives, which i think you call viscosity improvers, or better base oils.
I do not doubt that the sae40 works but i too have used multi grades for more than 10 yrs and have never had problems.