My owner's manual specifies ATF that meets Ford's Mercon spec. All fluids as far as I know that meet Mercon spec also meet GM's Dexron spec. Dexron is up to Dexron III, but Mercon is still Mercon. The exception is that there is also a Mercon V and Ford says it is not interchangable with Mercon and Mercon V should not be used where Mercon is specified. Personally, I don't agree, as Mercon V is essentually a semi synthetic Mercon.
As for myself, I use Mobil 1 ATF in my power steering which meets Ford's Mercon spec (it probably also meets and exceeds Mercon V spec as well).
In the industry there is a movement toward synthetic power steering fluid that is dedicated to power steering and not suitable for automatic transmission use. The Valvoline Synpower Power Steering Fluid is similar to what many automakers are using now as factory fill. I'm sure that it would work well in your car. As for me, I have chosen to use a synthetic that still meets Mercon spec.
