The pipe that has the oil in it - does it come from the vacuum pump or
the top of the valve cover?

If from the vacuum pump do you suspect a vacuum leak somewhere in the
car that's feeding excess air through the vacuum pump?

But, if from the top of the valve cover there's one possibility,
especially if the motor is driven for only short distances and so,
doesn't get hot enough to expel moisture from the engine oil. (Motor
should operate at 80 degrees C. If not, replace the thermostat.)

Inside the valve cover top is a baffle that's specifically designed to
allow any oil that's in the crankcase vapor to settle out and drip back
into the motor while the gases pass on to the air cleaner. If this
baffle is not clear the oil will be blown along with the gases into the
air cleaner.

Conversely, a 1978 car probably has done a lot of miles and its motor
could just be worn; that wear allows exhaust gases to "blow by" the worn
piston rings and into the crank case and then out the vent and into the
air cleaner. Higher rpms will push some oil along with the "blow by".
The definitive test for blow by is a compression test; that is the only
way the really KNOW the internal condition of a diesel engine.

My paper air filter element usually has a bit of oil staining and I've
come to regard it as "normal" so long as the air flow isn't diminished.
Spotting OK, soaking not OK.

What's the mileage on the engine?
After the diesels get up over 250K or so, they started having some
cylinder blowby caused by worn rings and cylinders. At that point oil
is blown into the air cleaner.

Doug