Brake systems (the hydraulic portion, not the friction portion) are filled with a fluid that is hygroscopic. The maintenance that they require is largely related to dealing with that fact, and the resultant water in the system.

What they have in common with transmissions is that they don't tolerate being overheated very well. Which is why race cars get new brake fluid very regularly.

Shock absorbers are a better example, although others have dismissed that one here before. They are hydraulic systems, and sealed. We don't need to change the fluid in them. When they are worn out, we replace them. The fluid is lifetime. One could argue that changing the fluid would make the seals last longer, but we haven't considered that to be an economically viable proposition since the days of lever action shock absorbers on British sports cars. And there, it wasn't that the fluid wore out, but rather that the seals wore out, so the shocks were overhauled.

I'd like to challenge that claim. This one is a bit of a round about reference, since the lubrication demands on transmission fluid are so low. But since the thinking ties back to reducing wear, and parallels engine oil change patterns, I think it is relevant.

In 2006, and again in 2007, SAE published papers demonstrating that in fact used engine oil was better than new engine oil. The engine manufacturers know all this. Yet some still promote 3000 mile oil changes instead of 12,000 mile, which is demonstrably harming their engines.

Link to the SAE article here: The Effect of Oil Drain Interval on Valvetrain Friction and Wear

I no longer have a membership or my copy of the paper, so the paper costs something to download, but the abstract demonstrates the point.

The point isn't to promote the concept of lifetime engine oils, since engine oils are subject to contamination from the byproducts of combustion, and resulting decrease in TBN (unlike transmission fluid) But the papers do challenge the traditional thinking that new is always better than old.

So then please exain how a simple fluid change solved my trans problems?

Most likely the fluid was out of spec. That is when it is worth trying a fluid change, when you have a symptom like you had, as an alternative to just opening up the transmission. As I mentioned in post 20. But that is corrective maintenance, not preventative maintenance.

So say the previous owner serviced the trans before any issue was apparent. Can you say for certain that the trans would have still had the same issue? Why not correct it before a prpblem occurs.

Its a 400 dollar service done every 2-3 years. That small amount can save you thousands down the road in possobke trans repairs. There is no benefit in not changing trans fluid and plenty of benefits to changing it. New fluid is going to always provide better performance than old. Why not spend 400 every few years to help avoid thousands in repair bills. Why is there a filter in the trans? Do you not think the filter collects debris and contaminants?

I guess to each his own. I personally would rather roll the dice after stacking the odds in my favor.

There doesn't appear to have been a downside to you changing the fluid. You restored the original functioning. What would have been the benefit to changing it in advance, on the off chance that it would have needed changing? You don't appear to have suffered any consequential damage.

The benefit in not changing the fluid is in not paying $400 every two years for something that doesn't appear to have any real impact on transmission life.

The transmission has an adaptation process to deal with varying fluid viscosities. It isn't wearing out mechanically. Why would new fluid be better, unless the old fluid was overheated and had broken down? Especially considering the data above, about how older engine oil lubricated better than new engine oil in controlled tests. New fluid is not always better.

Sure, the filter collects wear material. Not contaminants, the system is sealed. The only things in there are what was installed originally. The filter does precisely what it was designed to do, separate out and hold those wear materials, consisting primarily of friction material.

The issue we are debating is whether changing the fluid actually stacks the odds or not. Do you have any data that shows that transmissions with fluid changes last longer on average? Or that transmission failure modes on these models correlate with fluid breakdown? How do you calculate these odds?

Why not change the fluid at regular intervals to keep the fluid within spec?

So how long does a filter "hold wear materials" before the filter needs to be replaced? How long to the magnets accumulate metal particles before they need to be cleaned off?

You mean to tell me that a filter that collects wear items will perform as well as a new filter for the entire life of the transmission?

A new filter is going to perform better than an old filter. I saw my old filter after they removed it and it looked disgusting.

So maybe the fluid will last forever, but the filter needs to be changed and that requires replacement of the fluid.

And stop posting excerpts from an article on engine oil, you said it yourself, engine oil and transmission fluid are two entirely different animals.

You go right ahead and believe the "lifetime" claim on the transmission fluid/filter etc... I'll continue to change mine at intervals as it gives me piece of mind and COULD possibly save me major repair bills down the line.

Because you don't know that it is out of spec. With normal use, it won't go out of spec.

Filter is designed to last the life of the transmission. Magnets do not need to be cleaned off. If there is stuff stuck to them, great. That is what they are designed for, and where that stuff is supposed to be. Out of harm's way.

Do you think it was your fluid that was out of spec, or that your filter was partially plugged? Which problem did you fix? If the filter was so bad, you could have replaced it and put the same fluid back in. I agree that it is hard for people to bring themselves to do that, once they are already in there.

So you are saying that maybe the fluid will last forever? That is quite a shift in thinking.

Yes, changing the fluid could possibly save you money down the line. But we have nothing to base that hope on, except "because it seems right". On the other hand, we know it will cost you $400 every two years or so.

If youre against spending hundreds on maintenance every few years youre driving the wrong car. Like i said, you continue to run lifetime fluid and i will continue to service mine at intervals. I have always done this with all of my cars and i have never had a transmission problem.