Engine oil entering the vacuum system =
* Failing vacuum pump
* Damaged injection pump vacuum shutoff valve

Normal functioning vacuum system flow:

* Vacuum pump, removes air from the system, pumping air into the crankcase, (Good = 28 - 30 inches mercury), (minimum = 13 inches mercury).

* Check valves, and pump piston seal prevent engine oil being sucked into the vacuum system after engine shutdown, wear/age causes slow failure = small amounts of engine oil entering the vacuum system accelerating system failure.

* Damaged injection pump vacuum shutoff valve diaphragm, this circuit is in vacuum during and after engine shutdown = small amounts of engine oil enter the vacuum system, gumming up check valves, degrading all rubber parts, and accelerating system failure.

Engine oil is near impossible to clean out of the system.
Installing this filter on the shutoff valve is smart, adding others for climate control, and W123 central locking system can not hurt, and may prevent a large mess...