The lower wieght oils are for colder climates and are thiner. The Higher weight oils are thicker and perform better at higher temps.
In (northern) Canada people even run 0W30 oil. It helps when the temperatures extend down to -40 degrees for long periods of time.
I think us guys way down south can get away with using 15-50 oil, it protects better when it gets hot, and we don't need the cold temp protection (Texans don't even know the meaning of cold
).
I used to run sythetic 5W50 when I lived in Edmonton, Canada.