Re the last two posts, the "W" rating is what happens at the extreme cold end of a lubricant's performance envelope, there's a myth on BITOG that a "0W" will always flow better than a 10W at any temperature, which plain and simple isn't true.
Here's results from a series of engine tests, which shows the establishment of full oil pressure (FOPT), and oil flow from the rocker arms (RAOT, Rocker Arm Oiling Time) with different viscosities under different temperatures.
Take the SAE30 at 20F, 15 seconds to RAOT, 6 to FOPT. the 10W30 at 9F (11 and 6) and the 5W20 at -11F...they are similar times, offset by temperature commensurate with the "W" rating, or lack of it.
The SAE 30 at 30F shows what the engine was capable of achieving with full flowing oil...3 seconds to RAOT, and 6 to full oil pressure. The 5W does the same at 12 F.
If you increase the ambient to 50F, none of the oils are going to beat 3 and 6, the mechanical design of the engine will only move SO much oil through the air filled galleries, so the myth that a 0W at 100F will provide better flow than an SAE30 is just a myth.
Me personally, in Oz, have a current favourite viscosity grade, 5W30 A3/B4, which will do me anywhere in the country (I'd prefer a 10W30, but all of those are semis, and don't carry much of a cost reduction over the full synths).
OP, in pheonix, M1 10W30 would be great.
To extend on the "W" part, and it's extreme cold performance, it IS important to starting. There's a bunch of SAE papers that you can find on cold starting performance.
This one is good, as it's the limits of cold starting temperature for a range of engines, on a range of oils.
Note with interest:
* the 25W30 has a higher no start temperature than 20W50...demonstrating what the "W" means
* Similarly, the 0W, 5W, and 10W 30s indicate that in the OP's Pheonix, none of them hold an advantage in his ambient...the 25W would do if you wanted (I've run 25W70 to -7 or 8 C).
* The 4.0 straight 6 (I think it's Jeep) struggles at temperatures quite a way higher than the broad band of engines.