10 minutes online and I have my answer ...
What should be specified is
ISO 4925 Class 6. This is designed for
ABS / EBD / ESP systems, and is lower viscosity than any of the DOT fluids, although DOT 5.1 is probably close enough. And unlike a lot of the fluid spec alphabet soup, apparently this
does make a difference. It won't have any effect in everyday driving, but it might be enough to avoid a crash the day you need one of the safety systems.
I don't know about the specified Ford fluid, but I doubt that it meets ISO 6 as it is labelled Super DOT 4. The new Penrite ESP fluid meets this spec.
Also, WSS-M6C57-A2 has been superseded about 4 times by newer Ford specs, so don't go looking for it.