GM PSF is nothing special, from what I see, all ordinary PSF bottles state that they are approved for GM, Ford, and Chrysler products that use ordinary PSF.
The only types of PSF I see that are special are the Pentosin products that are needed in many German cars, PSF for Hondas, and MOPAR MS-10838.
Synthetic PSF will improve the ease of steering, and the life of the system. Royal Purple, Redline, and Amsoil make PSF that will do the job for your GM Saginaw box...
Oh.. and sometimes engine oil or tranny oil is used in them things... Might have been what the previous owner did.