All the books I've read, say DO NOT use ATF in a Hydraboost system. I have used Dexron, D2, D3, or brand X ATF, in stuff with vac boost. Seemed to have expected results, not matter which brand, or particular spec #.
Personally, I use regular old p/s fluid, and add some AT205 seal conditioner, if I have a lil escaping.
That's what I love about saginaw pumps. Run 'em out, fill 'em up, and keep rollin'. Run a Ferd pump dry 1 time, and it's junk. When I was working at the parts jobber, A-1 Cardone came out with a saginaw conversion pump, for the older Ferd pickups.