I agree with the degree of hesitance in using the WRONG ATF. That being said, here is my argument for being OK with using something like Mobil 1 or Amsoil synthetic.
There is ZERO incentive for the manufacturer to do the testing and approve of any other ATF not already on their list. Not only would this cost them money, they would get nothing in return.
I mean, if we are going by this line of reasoning, even Toyota's Type IV is NOT on the approved list, and technically NOT a suitable replacement. Heck, maybe it is the same basestock but with a different additive package.
Since Mobil 1 makes their own JWS 3309 fluid, this tells me they know what they are doing. They know the specs. As such, what reason would they have to clearly quote, black on white, that their universal synthetic ATF is suitable replacement for JWS 3309 if it was not suitable? Universal synthetic ATF are universal because they supposebly are better than the OEM specs of non-synthetic ATF. In Amsoil's case (or any other synthetic ATF manufacturer for that matter), they have zero reason to risk themselves to lawsuits from customers using their ATF that is the wrong spec. They do their own in-house testing.
Why not just use Mobil 1's 3309 fluid? Because it is not synthetic and the procedure to drain/fill the ATF is not fun on these trannies. I don't want to see myself in this situation 30K miles down the road. I want to use a good synthetic ATF that can hold up for 50-75K miles.