Yes it's the CDR, it separates crankcase oil mist back into the crankcase from having excess into the intake. If it's bad, you will put too much oil into the intake which usually makes cylinders 7 & 8 burn hotter, lift the head, could cause increase head gasket failures and "mystery" oil leaking from the head.
A simple rule of thumb is if the air cleaner bolt has oil on it replace the CDR.
First I have read about oil on the Air Cleaner bolt, to replace the CDR.
The CDR is simalar to a PCV Valve. The CDR is a pretty simple device, just a tuna can, a Spring and thin rubber diaphram in side.
They will always be oily inside, and you will always have oily residue inside the Manifold.
Cleaning them with a solvent too harsh can destroy the rubber diaphram.
I don`t believe they get gunked up or plugged up with crud as I have read on various threads.
Probably 10K threads on the CDR on various Forums, the good, bad and ugly along with old wives tales on them.
My first encounter with a CDR was when I bought a 83 Nissan PU with the Nissan SD25 Diesel engine, and it had this thing on the rear of the VC. What`s this piece of @$%t. took it off cleaned it with Carb Cleaner or Brake Cleaner, and the rubber turned kind of gooy mess. Pulled the rubber bits out with some needle nose and called it good.
My 82 Datsun Diesel KC with the SD22 only had a RDT. (82/83 is when the name change from Datsun to Nissan) When the engine was up to past 300K into 400K miles it was noticeable of the smoke coming out under the truck when stopped at a red light.
I wasn`t till I bought the 86 and signed up on FTE that I read about the CDR and what it was.
Here is a thread on Diesel Stop, think post #3 where the guy explains about the CDR.
The thread is about running the Blow By into the exhaust.
www.thedieselstop.com
All engines have blow by to some degree, just the nature of the beasts.
Goat