So.. finally i bought the second hand saildrive and i will try to repair the other one once out of the boat.
The guy that sold me the saildrive said to use only mineral oil SAE 30. So i will go with that, as it is the same as in the workshop manual.
I also called the Bukh distributor here and they didn't give me a clear answer. The guy said 1 out of 3 saildrives you will see uses different oil.