Thanks Ken2. All else equal the base oil should be thicker in a 20w50 and should get progressively thinner as we go down the viscosity scale: 15w40, 10w40, 10w30, and 5w30. But all else is not equal. It appears that companies use more lower group oil in the narrow spread oils (20w50, 10w30).
Also there is a lot of leeway to meet the spec for a given grade. For example, a 10w30 must ve between 9.3 and 12.5 cSt viscosity at 100C, must have a minimum HT/HS of 2.9, must have a max cranking viscosity at -25C of 7000, and must have a max pumping viscosity (with no yield stress) at -30C of 60,000. This leaves some leeway as to how to acheive the grade by varying the base oil viscosity, base oil quality (group), pour point additive, viscosity index improver additive, and other factors.
So the question of wht makes up a 15w40, or any other grade, can only be answered to satisfaction if you are the chemist in the oil company lab that actually formulates the oil (and that may include the chemist at the base oil company too). Unfortunately the oil companies are very tight with the data they release.
Three parameters that interest me for making oil selections and are very hard to come by are base oil viscosity used, viscosity index imprrover amount, and oil group used.