I put Mobil 1 ATF in the PS system on my 2004 Silverado, and someone in the service department at my local dealership did me a "favor" and flushed it out, and refilled it with whatever fluid is that GM dealers use.
The PS fluid that they use at the factory is absolute rubbish in the winter.
I politely told them that they better not have any intention to charge me for doing that (and they didn't charge me). Their excuse was that it was "unapproved fluid in a vehicle that was still under warranty", and they honestly thought that they were doing me a favor. They assumed that someone had accidentally done that.
However, it worked just fine up until that point.