Check the vac' hoses that go through the fire wall. Migrating oil from engine will do that .

Most Ignition Key Vac' Switches I have dismantled on 123 diesels have had lots of oil in them. The rubber connecting hoses perish and leak.

Audi 5000 vehicles had that problem too with vac' signal to ECU..... they installed a line filter in the hose.

Can happen with any type engine with vac' hoses.