Oil pressure looks good, almost too good...
 

Just did a fresh oil change (rotella 15w-40) and added an oil pressure gauge. Here are the results-
Cold Idle: 50 psi
Cold 2000 rpm: 50 psi

20 minutes after startup
Hot Idle: 30 psi
Hot 2000 rpm: 50 psi

Thought 1) Was 20 minutes long enough to heat up the oil? My temp gauge was showing 204 for 5 minutes before I took the hot readings.

Thought 2) These number look good, great even, but they seem way higher than what others have reported here. Is this just “survivorship bias” and folks are only posting numbers when they are suspiciously low or could something unusual be happening in my engine to cause higher than normal readings. We are talking about a 1999 with 172k here, after all.

Thought 3) I know 15w-40 is gospel around here and god knows I don’t want to start ANOTHER oil weight post but with pressure this good is there any advantage/problem with switching to a 5w-30 high mileage oil? I’ve had the truck for about a year and have only done 15w-40 in the 3 changes I’ve done, but obviously have no idea what the PO’s did.

THat is good, stay with the 15w40 and you will get 250k out of her like mine.

1) Coolant heats up much faster than oil.

2) "hot" is a relative term. To compare oil pressures correctly, you have to know the oil temperature.

Here are some real world values, I measured after a cold start (outside about 55F):
coolant temp / oil temp / oil pressure at idle
140F / 104F / 43 psi (coolant gauge on the cluster says "nominal")
188F / 150F / 25 psi
188F / 170F / 16 psi
188F / 185F / 14 psi
It took me 10 minutes of driving to get the oil temp to 185F.

This was my thought as well. I just idled mine for 20 minutes in the driveway. Tomorrow I can do a short drive and confirm/revise the results.

From a cold start, mine takes 10-15 minutes of driving to get the oil up to temp, At that point the pressure settles at19-20F at idle.

As an update, I took it for a drive (~30 mins) this morning and my hot idle pressure sits at 20psi, which I'm obviously pleased with. I'm primarily doing this as part of a diagnostic, my truck sounds "diesel-y" at idle when warm. No sound when I first start it up and then shortly after the noise starts, it's at it's "worst" when my coolant reads 180F and doesn't get any louder with any subsequent temperature increase. It's not terribly loud, as a matter of fact I only really notice it when I'm next to a building or something for the sound to bounce off of. It seems to go away with some revs and I can't really hear it from the top of the engine, so I don't think it's valve noise, but I'm not ruling anything out at this point.

Interesting. I have a similar noise from mine (embarrassingly, I was doing curbside pickup at a store recently and the kid working asked if my Discovery was a diesel...)

I made a thread about it and people suggested potential exhaust leaks. I haven't really had any time to dive into it at all, but there's a video in my original post. Is this what you're hearing?

That is exactly what it sounds like from exactly the position I listen from (its more apparent on the driver side). I did an induction cleaning earlier this week to deliberately make some smoke and I swore I saw it puffing out of the driver side catalytic converter so I took it in to a muffler shop that does free inspections this morning but they couldn't see anything (it was just a visual inspection, but I guess you get what you pay for). I'm going to try and do the "shop vac blower" exhaust leak test soon and try and confirm.

Awesome - keep me posted if you're able to track it down :)

Sounds like you have the normal Disco engine tick/tap. Given your oil pressure I would suggest it probably is a slipping liner. Have you put in a 180 thermostat?