Why is oil leaking into floorboard of Mercedes 300d?

If it is directly below the ignition switch, then you have a problem with the vacuum tubes on the switch assembly. Remove the panels around the switch and see which one is leaking. Replace the tubes.

I have a 1985 Mercedes 300D. The oil pressure gauge failed which caused engine oil to leak from under the dash. The car does not have an electric oil pressure sending unit, it has a pipe which is connected to the engine which is oil filled.