This discussion mirrors others I've read on several forums, and as Seeker points out, even among trained professionals there are completely opposing opinions, and knowing a bit about human nature it's actually not so difficult to understand. Let's try resorting to a mixture of common sense and logic.
First off, is it probable that two completely opposite outcomes can occur from the same event? Can changing the fluid (and I'll be using the example of a complete flush and fluid change) in an older transmission be both good and bad for it? Of course my initial reaction is no, it is either one or the other, but as I've said I'm no mechanic, but I do have good mechanical skills, and a some considerable experience over years of working with lots of different tools and machinery which can be applied towards deducing which is most likely the truth (at least as far as my knowledge can help me to ascertain ;?). :s
Backing up a bit, lets take a look at the characteristics of lubricants in general: 1)Reduces friction 2) Transfers heat 3)Prevents oxidation/corrosion 4)Carries away contaminants and debris 5)Protects against wear 6)Reduces energy loss/improve efficiency (as applied to mpg for instance).
Wikipedia says this about ATF's:
Most ATFs contain some combination of additives that improve lubricating qualities,[2][3][4] such as anti-wear additives, rust and corrosion inhibitors, detergents, dispersants and surfactants (which protect and clean metal surfaces); kinematic viscosity and viscosity index improvers and modifiers, seal swell additives and agents (which extend the rotational speed range and temperature range of the additives' application); anti-foam additives and anti-oxidation compounds to inhibit oxidation and "boil-off"[5] (which extends the life of the additives' application); cold-flow improvers, high-temperature thickeners, gasket conditioners, pour point depressant and petroleum dye. All ATFs contain friction modifiers, except for those ATFs... that specifically excludes the addition of friction modifiers.[8] According to the same leading oil distributor, the M2C-33 G specification requires fluids which provide improved shear resistance and oxidation protection, better low-temperature fluidity, better EP (extreme pressure) properties and additional seal tests over and above M2C-33 F quality fluids.
So now for the logic part. Which fluid possesses more of the aforementioned properties, new transmission fluid or old? Which is more likely to reduce friction, disperse heat, prevent corrosion and transport away debris and contaminants? Well I don't think it takes a genius to understand that old/dirty ATF cannot compete with new/clean ATF, so there's ZERO sense in the idea that the old fluid can in any way be better for your transmission than new.
So the next step is to consider if it is possible for new ATF to do actual harm to your transmission. Obviously it's not going to damage the actual components, but as far as I can tell one major argument is that the new ATF will break down sludge/varnish/contaminants, and these will do the damage, clogging orifices/lines leading to failure. On a certain level this sounds like it could make sense, but lets examine this more closely. This argument hinges on whether or not clean ATF actually does what proponents of this idea say it does, i.e. causes chunks or at least sizable enough particles of contaminants to break loose, circulate throughout the system and then create clogs that can cause a breakdown.
This is where my knowledge breaks down, I don't know much about the internal mechanics inside a transmission - tore apart the tranny on my '69 T-Bird when I was 19, and that was enough to teach me never to try that again (ended up bagging all the parts, putting them in the trunk, and practically giving the car away, lol). I have however seen how dirty an old, filthy tranny can get, but save for those really poorly maintained ones, I'm wondering if that's more the exception than the rule. :huh:
I do wonder if there's been any kind of study done on this - could you take some old sludgy, gunked up parts and put them in a bucket of new ATF, and then wait and see what happens? I could definitely see how a lot of it would dissolve into the new fluid, but will some more solid pieces break loose intact? Without being able to prove or disprove this theory, I'll admit that it's a possibility.
So if you were to take a sample of say 100 older vehicles, after changing the ATF in them the outcome will be that some transmissions perform well while others fail, that is a fact. I have no idea of what the ratio would be, but then that might not matter because since each and every one has been driven, maintained and overall treated differently there's really no way to be sure exactly what caused the failure, I would think, unless you could do a complete in depth "autotopsy"
But if it fails shortly after the fluid change, you can bet your bottom dollar that those people will be inclined to blame the new fluid ESPECIALLY if they (or more likely their mechanic) has heard of this theory before. In those instances it becomes a self-fulfilling proficy as it were, but it's also an example of faulty reasoning connecting two dots that may not have any relation to one another. A failed transmission after a full flush/change does NOT automatically
mean it's the fault of the change itself.
And if you took another 100 said vehicles that you didn't change the fluid on, while I'm certain more would fail than not, when they don't fail those people (or their mechanics) will be all too quick to point out that the reason they didn't was precisely because they DIDN'T change the fluid, using the same faulty logic. And for all those that did fail, well hey they were old and probably going to fail anyway, lol, so imo that's how these kinds of theories gain legitimacy. And while I could be completely full of bull manure
, all things being equal, logic tells me that given what we know about the advantages of new ATF versus old, that the odds are ultimately in favor of changing the fluids. Yes there may be risks with doing the change, but leave in the old and it's a certainty that you'll increase the wear and tear, reduce what life there is left in the transmission and increase the odds of a major breakdown in all likelihood. So that's my theory and I'm stickin to it, unless of course there are some facts that prove me wrong, in which case I'll gladly switch sides, lol.