On my 240D it was the gauge tube itself but sounds like yours may be associated with the line from the cut off valve or ign. switch hose attachments(If there is unwanted oil in the vac lines)...... Being that it seems to occur on the right side....
Real easy to see all of this if you pull the cluster again.....
The old pungent smell idea is ok too....kind of an all encompassing, warm, comforting, familiar, relaxing, Benz funk....they should make an air freshener with that scent....

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1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)