@Malleus that makes sense. No, I haven't seen one of the screens in person.
I've been contemplating what to do with my new to me LX (2013 with 93k miles). I'm on the schedule at the local Toyota dealership to have a full fluid exchange done with dropping the pan and replacing the screens. After this initial baselining, I think I'll just plan on doing the basic & drain and fill to keep it somewhat fresh and prevent any major buildup.
I've heard many say that you shouldn't do a full fluid exchange on higher mileage vehicles but I don't really get behind that comment. I'm still learning though. I had the full fluid exchange and screen replacement done on my 100 when it had 230k miles on it. Shifting was never bad but it was super smooth after and the average running temps dropped to the 135 range.
The dealerships make money selling parts. Replacing the screen won't hurt the tranmission, but it will drain your wallet.
The old wive's tale about not changing ATF is flawed; there are three primary components in an automatic transmission:
1) the clutches and steels - there are several for each gear, unlike a manual transmission which has only one clutch plate and one pressure plate and which are used to change from any given gear to any other;
2) the valve spools in the valve body, which are close fits in the bores they operate in; and
3) the fluid pump, which is a gerotor comprised of one external pump gear and one internal pump gear, in which the external gear turns.
The pump moves fluid into the valve body, where it is in turn moved into a section of the transmission containing the desired gear and applied to the pressure side of a piston, which in turn clamps the rotating clutch discs against the nonrotating steels (the steels don't rotate, but they can move along the long axis of the tranmission, as do the clutches).
The valve spools and pump rotors (gears) are close fits; any debris that gets between them and the mating surfaces they contact impedes their motion and potentially wears them or causes them to fail to move smoothly. The debris always comes from the clutch plate friction material that falls off when the clutch plates are squeezed against the steels, but haven't yet been clamped tightly enough to casue them to stop turning. This is what you want to remove by changing the ATF. If you leave that in long enough and other parts, usually the steels, start to wear and add to the contaminants in the fluid.
Failure to change the ATF, and subsequently remove the clutch frction material, causes improper operation of the shift or oil flow components; cluch wear causes slippage. This is why transmissions fail, typically (there are also failures due to component failure, but these are rare in Land Cruisers).
The clutch "packs" operate exactly the same way manual clutches do, except they are in an oil bath to keep them cool and to cause the system to operate (the transmission pump provides fluid for the valve body to use to shift the gears, in much the same way a clutch pedal does in a manual transmission).
This is why keeping the fluid clean is important, and why exchanging it for clean fluid when it gets dirty (contaminated) cannot in any way degrade the operation.
As to "changing" fluid by simply draining the pan, try this thought experiment: you are given the job of cleaning a dirty pool. Instread of draining the entire pool, you decide to use a bucket to remove water from one end, and, after emptying the bucket and refilling it with clean water, you pour it into the pool at the other end. How well do you think that'll work? That's exactly what you're doing when you replace only a portion of the ATF by draining the pan and refilling it.
