That’s a good point. Worth highlighting (chart from link below; I added arrows to clarify).15W-40's have a higher minimum HTHS requirement than 0W or 5W-40's. At shear rates even higher than those used for the HTHS measurement, there will be a larger difference, with the 15W-40 being much thicker. The W number is relevant to high temperature protection, even though that's not what it's meant for.
https://wiki.anton-paar.com/en/sae-viscosity-grades/