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SAE viscosity guidelines require 15w-40 grades to have a HT/HS of at least 3.7 Cp @ 150C. Most commercial engine oils range from 3.9 Cp to 4.3 Cp, depending on how they're biased in terms of high/low temp properties. The synthetic 15w-40s are more shear stable, so they can be formulated down around 3.9-4.0 Cp ....
When the CI-4 spec was introduced, most 15w-40 grades were thickened up a bit to compensate for the higher temps and soot levels of EGR equipped engines...