Adding to BH's response, there's a pretty nifty graphic from the Petroleum Quality Institute of America that shows a timeline of transmission fluid specifications that's helpful when trying to understand the history of ATF,
linked here for reference. Folks tend to have strong opinions here, so I'm not trying to argue what's "best," but rather provide additional background.
Fluid type (Type A, Type F, Dexron III, etc.) is based on a
specification, not a set formula, and different fluid manufacturers meet those specifications with their own various chemical recipes. Requirements center around various performance properties required for each manufacturer's transmissions such as: temperature stability, pour point, lubrication/friction-modification, anti-wear properties, viscosity, rust-prevention, and detergent content.
There are 2 main families of transmission fluid, and the primary way they differ is on the use of friction modifiers to create either low static friction fluids (GM/Dexron) or high static friction fluids (FoMoCo/Type "F"). Virtually all current fluids are of the low static friction type. As long as fluids fall within the same family (low vs. high static friction), it is quite possible for fluids to meet multiple specifications simultaneously.
GM has always, with very few (if any) exceptions, specified low static frictions fluids, dating back to 1949 when they issued the first Type "A" spec. Over the years as chemical and engineering technology improved, they've revised and improved the specifications several times, and up until recently, GM has explicitly stated that their new specifications
supersede all prior specifications. DEXRON III supersedes and is backwards-compatible (according to GM) with all GM transmissions back to 1949.
In the 1960's, Ford came out with the ESW-M2C33-F spec (commercially known as simply Type "F"), which specifically excluded the use of friction modifiers. Type "F" was designed to enable the shifting clutches to engage quickly, which was considered desirable as horsepower increased. All Ford transmissions were designed to use this different, high static friction fluid until around 1977, at which point new designs and revisions of previous designs began to require the use of low static friction fluids. Around the same time, lockup torque converters began appearing across Ford's lineup, and several various specifications were issued to both differentiate the fluid needs of these new transmissions as well as to address issues with these new transmissions within the first five years, culminating in the original MERCON specification.
Many MERCON fluids on the shelves today have essentially the same friction characteristics of DEXRON fluids, so it's now quite common for a fluid to meet and be approved against some version of both specifications.
It is important to note, relative to the discussion of DEXRON fluids versus Type "F" with respect to the proper fluid for an Ultramatic, that a transmission designed for a friction modified fluid (Type "A" or any of the later DEXRON formulations), like the Ultramatic, can use most Type "F" fluids but can experience harsh shifts or squawk when shifting. Conversely Dexron formulations should not ever be used in a transmission designed for Type "F," but as the Ultramatic was designed for Type "A," that shouldn't be a problem for us.
One more note: The main reason to stay away from fluids labeled Type "A" these days is that they are mostly made by off-brand manufacturers without any kind of quality control and may not actually meet the Type "A" specification, which is significantly outdated (
as evidenced by this recent PQIA bulletin). DEXRON-IIIH and the current MERCON-V specs both meet and far exceed the Type "A" spec and significantly outperform it in critical areas, such as temperature stability, oxidation resistance, anti-wear performance, corrosion inhibition, anti-foaming, and sheer resistance.
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