One of the obsessions I went thru was with oil. I did some writing on this, years ago, in an attempt to write a primer based on what I was learning. Be advised that back in those days I was wrong a lot. http://gress.org/Home/Cars/TrackStuff/DIY/OilingSystem%20Improvements/Oil%20Debate/The%20Great%20Oil%20Debate.htm
Don’t get too wrapped up into viscosity bands like xWy. What’s important is 100deg C HTHS (High Temp Shear) #'s. A high quality xW40 can easily have better HTHS #'s than a crappy xW50. HTHS #'s are available on the mfr’s website. Just be advised that oil formulations can change, and websites don’t always reflect the latest formulation.
Those viscosity’s are a “band” of visc #'s. One xW50 might be vastly thicker then another xW50. Or one xW40 might be almost as thick as a light xW50.
Heat and visc. Go look up a curve of oil visc vs. temp. You’ll find that visc changes rapidly when it’s cold, but flattens out when it’s hot. So oil visc doesn’t change much between, say 210deg and 240deg. A rule of thumb for hot oil visc is 10psi/10krpm. So at redline that’s 62psi. In a race car, don’t worry about going a little lower, but if you’re not getting 45psi at hot redline, then I’d put in heavier oil.
Oil visc drops over time. So your load or 10W40, after a couple months, is going to be thinned down to 5W30. This is a result of molecular shearing reducing the length of the molecular chains, and dilution. So when you top off, do it with oil a little heavier. Therefore don’t assume that 5W30 is inadequate for hot weather operation, it may be that it’s been thinned down to 0W20, or whatever.
The place to learn about oil is bitog, www.BobIsTheOilGuy.com
I do not run an oil cooler and I’m in the SE. As long as there’s lots of air blast on to the oil pan, the oil temps are ok. This requires some creative cutting of the skid plate.