the viscosity is already 5w through 30w as a blend, it would be fine to run year round.

if you were running straight weight, you would want to switch between 30w and then run 5w. the problem with that is in spring and fall you are going to have the wrong weight in there at some point.

here is the only chart i could find that shows two straight weights to the same in a blend..



see how the blend is good through the range, and the straight weight is only good for a portion of that range.
so if you wanted to run a summer fluid, you would need an air temp (not in the sun temp) over 104* BEFORE you step up to 40 weight.. AND you would need that to be consistently in that range for it to benefit you over 30 weight oil.