Yes, If oil is getting up into the Ignition Switch the Diaphragm is leaking in the Vacuum Shutoff in the IP

I have not read of anyone say the Oil damaging the Ignition Switch.

As a temp measure you can put a plastic fuel filter in the vacuum line between the Vacuum Shutoff on the IP and the Vacuum Shutoff on the Ignition Switch. That way the Filter will catch the Oil before it gets up their.

It is also possible you have more then one vacuum issue. It would be nice to check the Vacuum Pump vacuum all by itself to see what it is putting out.

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84 300D, 82 Volvo 244Gl Diesel