[Hydro]

Why were you "testing" the base oil in the first place?

 

[Hydro]

Have you tried to pour a little oil in the filter housing to help it prime?

That won't do what your thinking

The anti-drain valve in the filter base should not allow it
and the filter drain would have to be held closed

About the only way would be to pull the filter base off, exposing the oil port -- and pouring oil down that

The pump is likely worn or the suction tube leaking -- is why it wont prime
this is an 18 gallon pump anyways, so it should fill the filter base pretty quick ,even with the drain valve open

 

[Hydro]

Really the only meaningful test for the low pressure pump is done with a gauge -- there is one other that is not talked about much involving checking for air bubbles/frothing -- using a hose, valve and tee setup

The regulator valve(75psi) returns back to the pump inlet -- so when you took it out, the oil was allowed to drain back to the pan -- a weak pump likely would not be able to pull up from the pan

Side clearance is very important on a gerotor pump -- scarring is usually the damage(foreign objects)

If/when you put in a new pump, be sure to lube it before putting on the cover -- even some light grease is good, just no globs

 

[Hydro]

If your gonna try the old gears, smear gun grease on the surfaces and just a little gob between the gears, before putting the cover back on -- this should help prime the pump -- over filling the oil level will also help -- a couple of gallons, you can always drain some back out if the engine will start

I would suggest there is a lifter gone, but I didn't see the telltale marks on the gears -- I have had timing gears chip up and cause those kind of marks tho -- you can see down in the gears since you have the hpop cover off

With the marks and low base oil pressure, I would be concerned with the rod and main bearings -- those chunks should have got stuck in the filter, always a good idea to open the filter up and take a look at each oil change

Since the oil pressure "gauge" has been coming up slow, is when the metal damage happened -- but you say your 4th owner -- so hard to tell exactly when that happened

A real oil pressure gauge and fuel pressure gauge should be a requirement for every one of these trucks -- a lot of the usual problems could be noticed before there is real damage done