Let me clarify the viscosity thing:
When it is said a 15W-50 behaves as a 15 weight when "cold" and a 50 weight when "hot" - that is correct.
This does not mean it flows easier when cold or thickens when hot. It means that when cold its viscosity matches that of a straight 15 weight
oil - WHEN COLD. When hot its viscosity matches that of a 50 weight
oil - WHEN HOT. A hot 50 weight
oil will still flow easier than a 15 weight oil when cold.
The idea here is to not have the oil be so thick as to have poor flow properties when cold.
Maybe the best way to look at this is to look at only the second number. This is a measure of viscosity when the engine is up to temperature. All oils with a 50 second number will flow about the same (have about the same viscosity) when the engine is warm.
When the engine is cold you would like the oil to flow easily also. So then you wish instead of 50 weight, you had 10 weight oil. But if you put in 10 weight oil, it will be too thin when the engine gets hot... catch 22!
So now we get multi-viscosity oil. the oil is thin enough to flow cold (10W-...) and also thin enough to flow, but thick enough to maintain the hydrodynamic layer or wedge, when hot (-50).
Therefore 5W-50 or 20W-50 will flow similarly when hot, the 5W will flow better cold and in very cold regions may be advantageous. The 20W-50 might be better if you live in Phoenix.
Remember when you compare viscosity you must reference to the same temperature. 5W-50 does not get thicker (higher absolute viscosity) when hot, it just doesn't thin out as much as a straight 5W oil would when hot.
Hope this is clear as mud.
BTW 45 psi is just fine at cruise speed.
I would however suggest to most folks to build the motor tighter and run thinner oil. the thinner oil will take less power to turn the oil pump and the tighter clearances allow for less dynamic events to cause crankshaft deflection even when used with a thinner oil. Windage losses are also reduced. High viscosity numbers are kinda old-school or for full-blown racecars (2000 hp Blown alcohol can put a pretty big load into the bearings and can use some thick oil "cushion" but these engines are way shy of those type of loads). These days a properly built engine will work just fine with 10w-30
synthetic.