The unit is spec'd for GM Power Steering Fluid, and the part number listed in the manual is 89021184.
I won't run ATF in a modern GM steering system. After researching this quite a bit, it was actually on BITOG that I found out GM's PSF is a mineral fluid with a boron and phosphorous additives.
The last time I flushed out the system in February '13, and I've put about ~16,250 miles on it since. The steering started feeling a bit numb, so I thought I might as well change the fluid. When I disconnected the return hose going to the cooler and saw the stuff coming out, I wasn't happy to say the least. I went through 2 quart bottles of GM's PSF, but I'm going to be repeating it later on today to flush out the old stuff.
Given the condition of the fluid, I would like to switch to synthetic. I have little to no choice of product for an alternative, however. You get the OE fluids, like Honda's and GM's and so forth, but nothing in the aftermarket.
Pentosin and Febi-Bilstein have CHF 11S, 202 and 7.1 available in German parts stores and last I checked, 11S was a superior fluid to 202. I haven't looked into 7.1 though.