Hehehehehehehe,,,,,,,, that's somewhat ingenious think - using the compression gauge for checking the Oil Pressure. No, it won't rust !
By the sounds of things, you may at some point do well to take the Speedo Housing off and check all the connectors back there, perhaps even disconnect them - one by one - and clean them up. With the age of your Moke and that I'm pretty sure this hasn't been apart since leaving the factory, it's highly likely that some corrosion has set in. I gotta say, it's definitely not one of the most fun jobs, working in behind there.
It does sound odd for the Oil Pressure Gauge to behave in such a way. It appears the current range of these gauges read up to 140 PSI. I would think the sender that's supplied with it would match the gauge. Ordinarily, I wouldn't expect to see more than 80 PSI when hot and maybe 100 - 110 for a short time when cold, well within the range of that gauge. I'd suggest getting in touch with SAAS;-
I wonder if they have included a sender for a boost gauge here ?
Their Instructions and coff coff Trouble Shooting Guide, you can find here;-
but I would think that would have been supplied with the gauge.
A few checks you can do here;-
Check there is a good earth connection to the sender and also the green wire has a good secure connection all the way to the gauge. If this circuit becomes open circuit (a break in it), it will do as you describe.
If that checks out then disconnect the wiring from the sender unit. Check with a Meter it has between 11 and 15 ohms. Start the Engine and with warm oil, it should be around 75 - 85 ohms, cold oil around 90 - 100 ohms (unplug the gauge before doing this)
If that checks out, pop in to your local Jaycar store and grab one of these;-
Connect the middle terminal to Earth and one of the side terminals to the Green Wire (that runs to the gauge). Turning the shaft you should be able to have the gauge do a full sweep from zero to full scale and beyond.
These tests at least will help narrow down if it's the gauge, wiring or sender that's at issue here.