Charlie is correct, your '78 is lubed by the engine. And yes the earlier 4 cyl ones had their own oil reservoir. They had a plug on the side you would remove and then add oil through a cap on the top until fresh oil came out the hole. I think '76 was the last year for that, then they went to the engine oiled type like the 5 cylinders.

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H.D.B.
OBK #45
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'75 W115 240D, wife's dd, 197K
........'75 W115 300D, my dd, 245K
.. .. ..........'82 W126 300SD, the 'someday' project