Ford oils are made by Texaco;Castrol is part of BP.

Up until about 3 yrs ago, Texaco supplied all Ford oil. Initially all was badged Motorcraft, then some grades became Ford Formula (SD for instance.) Then BP got the eclusivity deal, and I think the Motorcraft badging was dropped in favour of more grades of Ford Formula. I find it a lot more expensive as a result.

In answer to OP, the bulk oil needs to meet the relevant spec. and I expect Oilman will be along shorly to criticise this piece......

Until they change it again !

But even so it doesn't mean that Ford/BP/Castrol or Motorcraft/Texaco lubricants are identical not even when they meet the same grading standards as the standards are simply maximums and minimums of the various test elements.

Now, how do I find out which refinery in Germany produces the Ford Formula Oil and who is the first dealer in the chain, so I can start buying from them?

It's very likely that the commercial agreements between refinery and Ford will actually preclude the sale of that exact specification to anyone else.

It's no different to own label food in a supermarket, which may be made by Heinz but to a different specification to Heinz-branded products - the supermarket own label product can only be sold to the supermarket!