If it is draining back oil (bypassing) will cause this problem. Either a cheap oil filter, oil, or the lpop is heading south. If you have a scanner check pressure vs demand on the high pressure side. To be honest, my e99 will start almost every time if u cycle the key (glow plugs) twice regardless of temps. It get down to 20 here in Texas not super cold but cold enough. Not repping or anything just my preference, but i run Delvac 15w40 and motorcraft filter, old red has 387k running original glow plugs, original harness, lpop hpop and injectors all original. Runs pretty decent for its age all 8 hit. With yours could be a ground issue to glow plug relay but the light doesn't seem to be staying on too long. Lot of little things can cause this.

In my 7.3 lifetime Experiences since 1997 and the 7.3s I own today... I have Never Been able to get one to start if that oil pressure needle won't move .....I usually will check the Injection pressure regulator sensor first . Than I'll move towards finding out if I have a broke a piston or Rings ...the low pressure oil pump is what everyone avoids because it sucks to change . More than the high pressure pump .. basically if there no oil being sent to or smacking the backside of those injectors its gonna be hard to start . But I'm just a hillbilly blabbing on about things did you ever find out what was wrong with it for us ?

Next time try jumping glow plug relay w screw driver. I usually hold it on about 45 secs. After see if it starts easier. Check valley for oil. Could be losing oil to a rail plug. It could be a injector ring. Not all show up with fuel in coolant. If you think hpop problem you need a scanner where you can watch live data and see what ipr icp and hpop are doing(demanding and creating pressure)

It’s probably injector stiction….. they have oil treatments that supposedly help, archoil is what I use in mine

Must have smell good

What did it end up being? Mine takes forever to start in cold too with lots of white smoke and I had GPR/harness/glow plugs replaced..

That’s injector o rings for sure mine did that, and if you plug it in and it’s fine then definitely o rings

158k damn near new mine starts perfect 20 degrees just glowplugs , good connections /batteries and starter. Mine is 300k and starts easy it’s also cause it has split shot injectors whenever they start to go replace them with single shot way more reliable and more tolerance can use motor oil

Did you also check cps camshaft position sensor weird it worked one day then next it didn’t

Mine does the same, im really looking for answer's

How are the hot starts?

Check your coolant reservoir if it smells like diesel fuel bad injector cups and probably could use a new low pressure oil pump should’ve had oil pressure way faster

Glow plug relay

Your glowplugs are not working, the voltage should be closer to 10v when they are running your gauge stayed at 14ish

Did you get a Silverado?!

Definitely injector o rings. Pretty obvious haha

Turbo pedestal seals are bad

I don’t know that it has an internal high pressure fuel pump. I know the 6.0. And the 6.4 have that pump but I’m not sure if 73 does however have you thought about glow plugs? That would be a good reason for it having a hard time starting when it’s cold