Higher weight oil in Hot climates?

Lots of start-ups require water-like viscosity. But you raise an excellent point - why not at least kick the first number up to say, five - in areas prone to being hot year round?

 

Thankfully, with synthetic the viscosity has allowed it to stay intact and useful at much higher temperatures.
I've used 0w20 in my 2008 Prius in 115 degree temps continuously without any problems, at all. Of course, I use Amsoil synthetic oil & filters. I have a 2011 right now that is 1 day brand new, so I can't speak for Gen 3 oil changes
I'm in Phoenix, AZ.

 

For crying out loud, we are talking about synthetic oil here, not the same as paraffin based stuff we used years ago.
Leave well enough alone.

 

There is no reason to up the 0W in hot areas. 0w gives you a bit better fuel economy when the engine is till warming up to full operating temperature than 5W does with no drawbacks in a synthetic oil.

If you wanted to muck with oil viscosity because of high temperatures, going from a 20 to a 30 would make more sense, but so far I'm not aware of any reason to do it on a Prius

 

I use 0w30 in my 2009. However since the dealer has to pay for the oil in my 2011 0w20 is good enough. 0w30 is my preferred oil!

 

No need the oil thats recomended is way better than higher weight dino oil. Plus if you went to a higher weight oil you will loose mpg.