Which PAS fluid for Fiesta?

Haynes manual says WSA-M2C 57-A, but I can't find this anywhere. I expect it's been replaced with a newer spec. Can I just get any old Ford (or any
other generic) PAS fluid, and use that? Or do I need an exact replacement?
Does PAS fluid ever need changing, or is it okay to just keep topping it up? I can't find a procedure for an actual replacement, so I'm thinking it's one of those things that doesn't need doing any more, like changing gearbox oil (for which there is no drain plug).
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Georg Von Krapp wrote:

Dexron 3 atf fluid is suitable. Change by draining the reservoir, refilling, running for a bit, repeat till it looks like fresh. or if you need to top it up regularly, it is self changing. Gearboxes oils can be changed used a suction device.
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Interesting, I already have over half a litre of Shell Donax (says it's Dexron III) lying on the shelf. I'll try that.
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