This is taken from the AMSOIL website:

"Remains Stable at Temperature Extremes

AMSOIL Synthetic Blend 15W-40 does not change viscosity (thicken or thin) at temperature extremes like conventional motor oils. With its advanced semi-synthetic formulation, AMSOIL Synthetic Blend 15W-40 is significantly more shear stable than conventional oils, retaining its viscosity at temperatures and loads that break down conventional motor oils.

Does this mean that I can run 15W40 in winter time since this oil does not change its viscosity?

later down the page it says "Pour Point -36 °C or -32°F" so does that mean the oil will remain same thickness down to that temp?

I know it may be a dumb question, but I am wondering now if I should run 5W40 in winter or just run 15W40 all year round since it suggests that the viscosity does not change