Well since the SD uses an electrical oil pressure gauge, It must be coming from the shut-off vacuum system. As the diaphram (at the rear of injection pump)leaks, oil is sucked through the vacuum system, which goes to the ignition switch. It eventually is pulled back into the motor through the vacuum pump.
Unfortuanately the rubber connections don't like the oil and usually swell up till they leak oil and/or vacuum. This occurs at the back of the ignition switch.
Bosch Master, ASE Master, L1
33 years MB technician