Agree every 5k is overkill.
But also note that OP is only changing ~40% of the fluid per a drain and fill. Does your 30k recommendation still apply in that case?
Absolutely.
First of all, let's acknowledge that Dexron IIIh (and the sister Ford product called "Mercon") are no longer licensed fluids. So there's no ability to get an "approved" fluid. That does not mean, however, that the current flavors of Dex/Merc, D3/M, etc ... fluids are "bad". Most of the major reputable sources out there still make decent replacement fluids for these outdated specs.
The main issue with these older fluids is that they didn't hold their viscosity very well. While they'd start around 7.3L cSt or so, they'd drop like a stone in 15k miles or less, down to 6.0 or even lower.
HOWEVER ... that does not mean they aren't doing a decent job of protecting the internal components (presuming there's no contamination present). I've got several hundred UOAs of different trans fluids in my database and I can say with certainty that even though these fluids do lose their vis, they generally still protect against wear decently when changed "regularly". I define that word "regularly" as 30k miles or so. The wear rates are generally NOT affected by the loss of vis when the OCI duration is less than 30-50k miles (depending on use factors). The data does not lie; there's no need to change trans fluid every 5k miles ... that's just throwing away money and effort. Seriously; it's a waste and there's no other way to reasonably look at it.
And as far as those internal filters go ... Change it once every 100k miles or so. These things are just basic rock-catchers; they are finite polishing filters by any stretch of the imagination. Since the OP had the trans rebuilt a few years ago, and the internal filter was likely done then, that's all it should need in the next 20 years or so of his annual 5k mile use. Seriously; these things are really porous and don't catch much at all unless there's a catastrophic failure, and well, by that time it's too late anyway.
Over-maintaining something has never proven to help equipment. It may satiate the brain, but it does nothing to extend the life of equipment. Once you have a good maintenance plan, doubling or quadrupling it does NOT mean things are going to get 2x or 4x "better".
For these older types transmissions (TH350, TH400, C4, C6, A727, etc) just do this:
- fluid every 30-50k miles depending on severity of use
- internal filter once every 100k miles
