Lots in the archives on this subject - e.g. [xj40] Green blood
In short, on your '91 with the Teves brake boost system, once the SLS and its troublesome hydraulics is removed, you only need the original HSMO oil reservoir for the power steering - and you may as well leave the mineral oil (‘green blood’ ) in rather than flush it out to replace with PS fluid because it rarely(!) leaks.