I,d recomend getting an aftermarket sender and gauge. they are easy to install and alot more accurate then those factory ones. I went with a Koso digital gauge kit. only trouble i had putting mine in was finding a short adapter to go from metric npt to us npt for the sender.
You can use the same guage for any truck of the same vintage. If you can find the guage cluster from any truck in the same series, you can swap the oil pressure guage with about four screws.
I've got a gas guage cluster, with a diesel speedo (so it has the T-belt light) installed that I'd be happy to part with, but I'm sure shipping to your neck of the woods could be prohibitive. Especially since you can just go to any junkyard or scrapper and get what you need. Basically, take the guage cluster out, push the tabs apart to separate the gluster from the clear lens, and then take a screwdriver and remove the two screws that hold the oil pressure guage in there out. Once you've got the guage cluster out (which necessitates removing most of the interior) it's a ten minute job, and that includes cleaning it all up!
FWIW, the same guage worked for the 22R-E and 2LT-E for me, so I'm sure it'll be the same with your 3L (the oil pressure sender is the same)
There is nothing nicer than a properly operating OEM gauge. It was designed to be easy to read and be informative without being too distracting. I can't stand these aftermarket gauges because you can't put them where they belong (in the dash) and are distratcing with their very different look and lighting. More distracting than informative. I'd replace the OEM gauge before installing an aftermaket one, which I would only use for calibrating.