About which spec to use, I'm confused.
In the past, I've just used exactly what went in at the factory. I've never tried to "upgrade" my ATF fluid to some other Universal or alternate spec.
Something about elastomer compatibility, frictional characteristic, viscosity hydrualics dependence? I just don't know. Its complicated, maybe.
I'm that way with spark plugs too. Just use exact factory ones there.
Motor oil and diff fluid is different, I'll use an "upgraded" spec as long as it encompasses the lower spec. For example, if only SN is required, its easy to find a motor oil that is both SN and other higher specs.