Years ago, I was told to syphon out the old diesel diluted oil that was in the pump before adding new oil. Seems to make sense that you would not want the diluted oil in there any longer. The new oil would not be able to magically displace the old stuff, especially with the only drain being a hi level overflow.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150