You have lots of options, but I don't know if the PS pump will like some of them. There is racing ATF available, which might be thicker. There is also regular PS fluid, but whats the viscosity? You could also try any of a multitude of hydraulic fluids. I would give someone like Redline Oil a call and ask the chemist. They sell almost all that stuff and could probably give you a decent answer.

Power steering pumps aren't very discriminating, btw. They'll be OK with almost anything.