The friction modifier in the 3309 reacts with something in Mercon V fluids is what I have hear. Destroys the clutches.
3309 is mineral based and highly friction modified, it can be used in place of Dexron-III but not the other way around, Aisin transmissions are picky.
Chrysler transmissions, especially from the ATF+3 and A604 issues onward, are also very particular about what they will and won't work with.
I would not doubt Mobil making it for everyone, although when I looked, Ford says their fluid was made in Canada. Castrol has a big blending plant in Toronto, and Ford does a lot of business with BP, so I was thinking that, or maybe Petro-Canada who makes all of GM's factory fill transmission fluids for Dexron VI now. Does anyone know where the Mobil 3309 is made? Esso sells it in Canada as Esso 3309, so it could possibly be made here, as well.
Anyways, with so much 3309 spec'd fluid out there across the parts counter or from a Mobil distributor, I would just use the right stuff and not worry. How often do you actually work on your transmission or how much money do you have to put into it for it to be a really big deal? Transmissions aren't cheap and considering how little maitenence they take to keep reliable, might as well use the specified stuff and forget about it.