I have added a Magnefine to 2 of my vehicles and a full flow spin on filter onto my Truck.
If you shop around you can also add some high temp neodymium magnets to your transmission pan to help catch more materials.
I like both magnefine and spin on systems.
My opinion is that you should either get a Magnefine and change it out after 5,000 miles, then you can probably go 15,000 to 20,000 on the next. Or take a look into the full flow filter setups and change that out at a similar interval.
Your rebuilt transmission will produce something like 90% of its break in materials over the first 30,000 miles. However the vast majority of that will be within the first few 1000.
If you do get into aftermarket filters just be sure to check it regularly and change it out occasionally. Damage can result by forgetting about it. Hence why Ford no longer "authorizes" their use. They have to be changed out every 20-30,000 miles and it seems that most people drive a car, even with idiot lights on, until it dies.
(However, being a Member of BITOG I seriously doubt anyone here has that issue.)
Just realized I didn't answer your question.
If they are using a Dex/Merc I think that you are safe to stick with any Dex/Merc fluid. There are many out there that would fit the bill. However, Castrol Dex/Merc is pretty good stuff. I've done research and I haven't ready any complaints on it.
I've started researching the fluids currently carried at my Walmart and that is one of the non-Supertech brands they carry.
I think it is a very wise idea to change fluid at 3k post rebuild, as it will clear out a ton of metal shavings that will end up in the fluid after a fresh rebuild.
