Is used M1 back when it only said "Dexron III/Mercon" and I got excellent results. The newest M1 is still fine to use in a Dexron III/Mercon system, however, they are advertising that it goes almost anywhere. I'm sure it is still a great product.
Allison transmissions that were attached to Duramax engines stated that Dex III was okay if changed frequently. HOWEVER, there was a fluid standard they created called TES-295 which extended OCIs for the transmission and extended the life of it. Putting it in an ordinary DexIII transmission is perfectly okay.
Now.... What stores have Castrol TranSynd on the shelves? NAPA, CarQuest, Pep Boys, Autozone, and Advance don't have it on the shelves, but other guys might. Maybe the Florida Panhandle has stores not found in southern and central Florida?
In any case, M1 ATF is priced between $6.50-$8 everyday at most stores, and it is usually in stock.