In my yrs I've done a fair amount of research and testing of oils. Most important is what I observe when I've rebuilt my own engines (usually for increasing strength / power) as well as observing sounds and MPG as I try different things.

I'd suggest everyone with an older ford engine use a "HM" (high mileage) oil. This includes the modular series of V8's. You'll notice a HM oil is a little quieter and it can help condition seals.

On my 98 3.0L w/153k miles I run 10w-30 mobil 1 HM. Its quieter than ford 5W-20 semi-syn after warm up and I've noticed a bit of a decrease in MPG. (the oil is more viscous and consumes torque to pump the higher viscous oil)

I've not added to my ranger yet, but often times on a 150K+ engine I'll add 1/2 a bottle of regular STP or HM STP oil treatment. That additive has zinc in it & will also quiet things a bit. I've seen it greatly quiet lifters on a 300k mile buick. If the engine is still fairly tight I might keep using 5w-20 but add 1/4 or 1/2 a bottle of STP. I try to add just enough until I can hear that its quieter after full warm up. (20+ miles of freeway driving)

I'd only run 20w oil in a newer engine w/less than 100k+ miles.

Here is a link to help understand how its tested -> https://www.machinerylubrication.com.../oil-viscosity

 

Yes, +1 on above

The second number in 5W-20 is its warm viscosity, the lower the number the "thinner" the viscosity, so 5w-30 would be a "thicker oil" after it warms up
Oils viscosity is thicker as it gets colder and thinner as it gets hot
The first number, like 5w, is the viscosity of a 5w at a COLD temp, so viscosity is not 5, the oils formulation makes it act like a 5w at 0deg instead of a 20 at 0deg, a 5w would be thinner than a 20w at 0deg.

Oil pressure is the oil left over as oil is pumped thru the engine, the oil pump pumps oil to the filter and then it goes out the Main passage to the engine, where oil pressure sender is, the oil pumps capacity must be more than engine can ever use at any RPM or it won't work.
So the oil backs up in the Main passage, thats oil pressure, the oil the engine can not use at that RPM

A thinner oil can move thru the passages and bearings faster than a thicker oil, newer engines will have smaller gaps in bearings, less wear, so a 20 would be fine and oil pressure would still be high
As an engine wears the gaps get larger, so the thinner oil flows out faster and oil pressure will go down, this can cause rocker/push rod tapping, because these are at the top of the engine so they are the first area to be effected by lower oil pressure
Running a thicker oil gets back the oil pressure, high mile oil is a thicker oil

At 150k I would switch to 10W-30, not sure why 5W would be speced for southern calif. unless you go skiing in the mountains for weekends

 

Here in mid-ohio I run 10W-30 yr round. When I was up in Michigan I used to run 10W-30 as well but a few times each yr it was pretty sluggish to turn over and on one car it even screeched for the first few seconds. It was a 2000 Pontiac 3.1L V6. If I'm ever again living in a climate that will see below 0F temps I'd run 5W-30 during the winter. And if I was in upper Canada or Alaska I'd run 5W-20 (all being Mobil 1 full synthetic)