Winter range will ALWAYS be the worse of the two oils mixed due to wax formation, and the operating viscosity is a simple mathematical calculation that will almost assuredly be in the 20 grade range if there is more than just a “splash” of 20.It’s a range. Not a tally. You ever hear of a 2.5 winter rating or 25 viscosity before? You’ll end up with pretty much an undetermined winter rating between 0/5 and either a higher range 20 or lower range 30.
It’s not rocket science; it’s much closer to color-by-numbers. It’s one or the other, there are no arbitrary in-betweens.