Low oil pressure at idle

[BikePilot]

I'm picking up a 99 F450 for my dad (2/99 mfg date). Found one that's just what he wants (4x4, classy chassis conversion with fancy interior and pickup style bed, 7.3L diesel of course, no rust, 6spd manual) and is cheap (for a reason).

The problem is at idle the idiot gauge goes to zero and the light comes on, anything above about 1100rpm and the gauge pops back up and light goes out. It seems to start and run fine. It might crank a tiny bit longer than normal, but I'm not sure. This would be our first PSD. It cranked a touch longer than my IDI 7.3 turbo - maybe 2-3 seconds before starting when warm. Haven't seen it cold yet.

The truck has 91k miles on it, oil level is correct and the seller has receipts for regular oil changes at a local ford dealership (15w-40) for the last 30k miles or so. It was used to tow horse trailers around by the seller who was the second owner and bought it with 60k miles on it.

I'd guess first order of business is to hook up a mechanical gauge to verify actual oil pressure. Any other thoughts or likely trouble areas?