What ATF components wear out or are displaced over time?

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I can't begin to understand the oil lab reports; wish I could!
Regarding ATF, there are those who believe in services and those who believe in lifetime fluids and those who don't care either way.

I believe there are detergents to remove contaniments and keep things clean; these contaniments are suspended in the fluid or form sludge, right?
So, what gets depleted? What gets worn? I think viscosity properties must be diminished, right?
I understand heat is a tranny killer. I am guessing the fluid gets burnt which leads to poor lubrication and transmission failure. Please correct and amplify for me.

I appreciate your thoughts.
Extra credit: How often do you service your automatic transmission?
 
Clutches wear

O-Rings develop cracks and lead to pressure loss, which leads to clutches that don't apply properly.

Bushings and bearings wear out.

Solenoids can develop varnish which can change their apply rates, leading to slipping and more clutch material in the fluid.

Filters can plug over time.

Extra Credit

My Honda's get a S&F every or every other oil change. The ranger gets S&F every 15k or thereabouts. Although it will get a filter change early if I get the aluminum pan for it. (More of 'when' than 'if' on that though.)
 
I'll skip to the end. On the Accord it gets serviced annually. It is a super easy drain and fill procedure. The Jetta is new and I plan on having VW do a drain and fill every 20-30K.
 
I initially went 127k on the factory fill because every shop I took it to made it out like it was impossible to change it because it was a "sealed" transmission. Once I started doing my own oil changes and realized how easy it was I now change it about every 20k. Heat is the enemy of a transmission because it breaks down the fluid, synthetic is much better but virtually any fluid subjected to extreme heat and pressure will eventually breakdown, that's just chemistry and entropy. Unless you don't intend on keeping your car past 150k, no fluid is "lifetime". When the tranny takes a dump that's when it's lifetime is up. You can smoke two packs a day and drink 12 beers a night and live a full life to the age of 55.
 
Basically what ctechbob said, but I’ll throw in any friction modifiers as well.

I’ve yet to really own a vehicle long enough to “require” a fluid change. I did one on my Durango, but I bought it used at 55k miles and had it changed at 105k only to trade it in 4 months later. My truck will likely get one around 50-60k per ZF’s recommendation (Ram claims lifetime).
 
To answer your question, what gets depleted in the transmission fluid, I don’t really know...could be the viscosity itself, but the culprit may be more contamination...clutch material, whatever forms from heat degradation in the actual fluid....things that do eventually find their way in (salt/silicates/sand/dirt/pieces of bugs/whatever). I have seen some things in pans that you wouldn’t expect to see in there. Little different colored specs of whatever.

I generally change my fluid once a year, just simple drain and fills. Really just fluid replenishes. I know by doing this I’m constantly replenishing the additive packs and removing some debris. I’ll usually change my filter initially upfront my first service...say, at 40,000-50,000 miles. And then I won’t bother with the filter again. Same for the magnets in the pan...I’ll drop that pan once initially and clean the magnets off, but I may not ever do it again. Sometimes I’ll install a magnetic drain plug...figure I’m helping the other magnets out.

As for the filter...there’s some interesting informative videos out there about just how hard it would be to actually clog those filters up (or for stuff to get past them). When cut open the filters are pretty big, actually. And even after 100’ of thousands of miles, there isn’t a ton in there and certainly not enough stuff to be in danger of clogging anything. I’ve cut open a few myself and really there isn’t much in there (it’s nit like an oil filter in an engine that is dealing with an internal combustion engine and all that soot and byproducts of combustion). Especially if you’re taking care of that fluid and draining and refilling it annually. Or every 50k or so. Just my opinions here, but it’s also what I’ve done and I’ve taken some very suspect transmissions up to almost 300,000 miles without issue (before selling them). Knock on wood.
 
It's probably not so much the fluid/lubricant that goes bad but the depleted additives and contaminates that are circulate in the trans are the reason for changing it.
 
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