Originally Type A automatic transmission fluid was used which has been superceded by the Dexron variants.
I use Dexron II in the power steering systems of my Chevelle and Camaro. Above link in the 2nd post also references:
"The GM part number for a quart of power steering fluid is 89020661. The previous GM part number for power steering fluid was 1050017. Both fluids work equally well."
The over the counter stuff at the local parts store may be fine, a lot of people use it, but given the age of our rides, sticking with Dexron ATF or the approved specific GM fluid makes sense to me.