There are good full visc synthetic ATF's that meet/exceed DexronIII. There are also unlicensed fluids that could have more 'compatibility' with multiple makes. I don't blindly use the latest greatest without reading the bottle label and manufacturer datasheet. DexronIII isn't licensed anymore so your are relying on bottler reputation. I don't use ANY conventional dex/merc, even from Toyota parts. Thats akin to looking for 20 year old bottles of API SG oil at the podunk town autoparts because you don't want to use a newer API SP/GF6 oil.
I would not blindly grab a Dexron VI from the GM dealer and add it to ANY non-GM product. And, I wouldn't use it in place of differentials, power steering, transfer cases, manual transmissions, or non GM automatic transmissions, without the bottler mentioning those automakers and components. Don't assume that the Dexron-VI can just blindly be used anywhere.
If I had a Toyota requiring DexronIII, I would use Mobil1 ATF, RedlineD4, or Amsoil ATL, and obviously and equivalent boutique competitor. I don't care for the pathetic performance of conventional ATF. Another good list of 'meets or exceeds', or listed suitable for 'dexronIII', by various brands, are the Allison TES 295/468/668/389. Some brands will state suitable for, meets exceeds.... the older specs. Others don't. This brings in the universal/multi-spec/multivehicle non-licensed ATFs that some love, and some love to hate.
The problem isn't with PSF's only. Did GM not come out with a new MT fluid for those manuals requiring DexronIII too, and specifically stated NOT to use DexronVI in those manual transmissions? So, backwards compatibility can NOT be assumed for any non-GM product, or even non traditional automatic transmissions, or Allisons that haven't been rebuilt with upgraded seals, or YOUR PSF, without the blender label/website or automaker specifically stating so.
Allison Transmission approved fluids are tested to meet our stringent requirements, which ensure maximum transmission durability with minimal downtime.
allisontransmission.com
Not licensed but recommend for:
Discover how synthetic automatic transmission fluid like Mobil 1™ Synthetic ATF provides your SUV or car outstanding resistance to oil breakdown and deposits.
www.mobil.com
Any mention of Dexron anymore:
Mobil Delvac 1 ATF is a fully synthetic fluid recommended by Allison Transmission, Inc. approved against the Allison TES-295 Specification.
www.mobil.com
Mobil Delvac 1™ ATF 668 is a fully synthetic transmission fluid recommended by Allison Transmission, Inc. that is approved against the Allison TES 668® specification, and is backwards compatible for applications recommending Allison TES 295®.
www.mobil.com
When looking at Valvoline Maxlife ATF, because of pricing over the years, became overhyped super popular, does it EVER MENTION manual transmissions, power steering systems, transfer cases, or differentials, on the website or datasheet??? Think about how so many have been brainwashed and use it as steak sauce and wafflepancake syrup too. Luckily, Valvoline made it very mechanically(and not biologically) compatible for those that used it where it wasn't recommended. I would wager that a PSF, from those boutique manufacturers, would also have similar compatibility.
I would not blindly think that a licensed DexronVI does, or can be used everywhere blindly. My Toyota's did not call for a DexronVI or LV ATF their automatic transmissions, power steering systems, or differentials. So, I would recommend the full visc ATF, and a full synthetic upgrade when/if in budget.
This is beyond what the threadstarter asked..... synth boutique PSF vs unlicensed conventional DexronIII.... I would take the synth PSF.
Doesn't even mention ATF or Dexron but can be used where:
Prestone® Asian Vehicles Power Steering Fluid enhances system life vs. conventional fluid. This synthetic fluid is designed with 2X Anti-Wear System Protection, which means more protection against pump failure, squeals and clogs.
prestone.com
Pretty sure over the years that MotorMedic, Gunk, Bardahl.... also had Asian PSF's that didn't mention 'dexron' either.
And then you have this PSF, for use where all those ATF's were originally spec'd, but not in your automatic transmission:
www.lubegard.com