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We had a long debate on some national (e.g. French) forum and came to the conclusion that oil o use with old engine has to stick to API SFCD 20W40 norm
This was recently explicitly confirmed to me by french Ford dealer Fornault. This is also explicit on the Ford 1975 documentation (“Oil class API DS(CD) or range III”)

The idea behind that being the modern oils (API SFCF or higher) for modern diesel engine exhibit a too high TBN (Total Base Number) for those venerable old ladies.
This is related to sulfur proportion, lowered in modern gasoil.
SFCF has a TBN of 12 while SFCD as recommended by Ford has a TBN of 4. As a backup one might consider API CF-4 with a TBN of 8

I found a paper (inj english langage) related to this and the TBN here :
The conclusions of the paper where found acceptable by the audience of this French forum

The curious side effect of this SFCD recommandation is that I now have to look for oils in cheap popular supermarkets and flatly ignore whatever oil is dearer than 10€ for 5 liters. Not always easy to find.