The main reason to change the oil on these older IP's was to eliminate the diesel fuel dilution that occurs over time. If I remember correctly, there is no drain plug, you could siphon or suck out the old stuff though.

Seems like I was told many years ago to drain the contents down to the check level plug then replace the plug and refill to above that point. Repeat a couple of times. This was very messy and a number of years ago on the old '68. Supposedly the new oil is more dense than the diluted old oil?!? With todays understanding of these devices, I would go with the syphon style removal of the old oil.

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