Been there, done that on my sister's 300d. As you might have noticed, the mechanism that moves the needle is kind of a metal bladder (bronze i think) that expands as the pressure increases. Depending on where the leak is, you might be able to solder it with rosin core solder (the one used to solder electronic components).

Just disassemble the pressure gage by removing the back cover of that segment of the cluster. Remover the oil pressure needle by gently pulling it with some pliers. Clean the metal bladder with contact cleaner, locate the source of the leak and apply the solder.

It's very likely you will be able to fix it. Good luck and god speed!

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1982 W123 300D NA Euro, 300K Miles, 4-speed manual, 3.46 rear diff, early W126 front brakes, Penta rims with 205/60R15 FH900's, custom fitted (by me) audio system, more to come into the money pit... Soon to be installed: Bilstein HD, R107 front discs and 4-piston calipers, HD Suspension springs, wagon front swaybar.