5w30 or 0w20 in Gen 2 Prius in CO? Mixing viscosities?

I did find this one here:
Mixing oils of different viscosity...? - 600RR.net

"All mineral oils are produced from the same basic petroleum feedstocks. Different oil viscosities will not separate or react negatively to each other because the base oil is molecularly the same. It's the different oil additive systems between brands that shouldn't be mixed - if you're concerned about the oil performing exactly how it was designed.
Oil additives include:
Anti-oxidants to prevent thickening at high operating temperatures.
Pour point depressants which lower the temperature of wax coagulation
alkaline materials to neutralize acids formed during combustion.
Rust and corrosion inhibitors.
Detergents to reduce sludge and varnish.
Dispersant additives to hold contaminants in suspension.
Extreme pressure additives to prevent metal to metal contact under high loads.
Viscosity index improvers in formulating muilt-grade (viscosity) oils.

The exact type and amount of these additives varies between brands and when mixed may not function as efficiently as desired. Different viscosities of the same brand oil will have different amounts of viscosity index improvers (polymers), but otherwise the additive systems will be the same.

From the Chevron.com site:
"Can I mix different viscosity grades of motor oils?"
"Yes. It is always advisable to not mix motor oil brands, however, different viscosity grades of the same brand motor oil are compatible. Be aware that mixing viscosity grades will turn out a product that is different in viscosity than either what was originally in the engine or what was added."

From Shell.ca:
"If you mix viscosity grades such as a 5W30 low-viscosity oil and a 10W40 higher-viscosity oil, it is reasonable to expect that the resulting product will have viscosity characteristics which are thicker than the 5W30, but thinner than the 10W40. This change does not reflect incompatibility - it's simply a re-balancing of the viscosity characteristics. In all other ways, the product should work as expected. But there's absolutely no danger about incompatibility resulting from mixing engine oils," Miller says. "We've tested all of our grades and brands, and we haven't observed any problems."

From Mobil1.com:
"For our customers to choose a viscosity grade, we recommend they follow the engine manufacturer's recommendations as indicated in their owner's manual. There is no need to mix two Mobil 1 viscosity grades when one will do; however, we see no problem mixing different SAE grades of Mobil 1 Tri-Synthetic series motor oils.' "

 

1. This would be fine if you were using the same oil brand. If you are mixing brands, that might not be optimal practice.
2. 0W-20 is recommended for 2010 and newer Prius and the oil change interval is 10K miles. Personally I am not comfortable with that interval and change oil at around 7K miles.
3. If you change oil twice a year, considering the wide variation in ambient air temp, I would suggest using 0W-20 in the winter and 5W-30 in the summer.