Vac Diaphragm leaking
If you will do a search using my user name you will find all my posts on my '80 300TD vac pump diaphragm change out and how it fixed the oil usage problem. The parts can be bought through Fastlane for less than $20 and the fix is sure worth it.
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan