Take a look at a few 5w-40’s and their kinematic viscosities, viscosity index, ZDDP ppm, and sulfate ppm. Most I've see are as good or worse than 0w-40.
In example, if you look at the Mobil 1 0w-40 (FS in specific, also to the ESP) and compare it to:
Valvoline 5w-40 MST (Dexos 2)
-Kinematic viscosities are nearly the same.
-The VI much lower
-HTHS is nearly the same
Motul 5w-40 8100 (Dexos2)
-Low temperature kinematic viscosity is higher, but high temperature kinematic viscosity is nearly the same
-The VI is lower
-HTHS is the same
-ZDDP is unkown
-Sulfated ash is lower
https://www.motul.com/rs/en/products/8100-x-clean-5w40
Mobil 1 5w-40 Turbo Diesel (non-Dexos 2)
-Low temperature kinematic viscosity is higher, but high temperature kinematic viscosity is nearly the same
-The VI is much lower
-HTHS is nearly the same
-ZDDP is the same/nearly the same
-Sulfated ash is nearly the same
https://mobiloil.com/en/motor-oils/m...o-diesel-truck
I 100% agree with you from a theoretical point of view. When you look at the test qualifications, it leads me otherwise.
Remeber, VI and VII are related... it depends what kind of VIIs are used.
900+ppm ZDDP in a gasoline engine formulated oil is pretty damn good these days. Yes, going to a multi-applicable/diesel oil gets you more elevated ZDDP ppm, but you need to consider a few things: NOVACK and sufated ash; then whether youre going to run a catch-can.