What if the 'someone' was the owners manual, that was written especially for your year and model Mercedes Diesel? The one for a '68 220D/8 specifically states to use ATF in the manual trannie. Mine (see below) was still working fine when we ceased driving it for other reasons, and it had ATF in it the whole time.

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