It's just regular GM PSF that is specified for the Magnasteer racks. I don't know of any special characteristic of GM PSF.
My understanding of the Magnasteer system is that it's based on electomagnets interacting with permanent magnets, with no fluid involvement at all (unlike the older variable fluid orifice design). Someone chime in if I need correction or clarification.
Some people on here are using high-end ATFs in GM power steering systems, while others argue against doing that. No one has been able to prove anything in that argument.
I have gone the ATF route myself, for the purpose of reducing garage inventory. I have a Magnasteer rack, and it's worked fine so far.