The fluid and the filter have many critical characteristics. They don't vary linearly, or necessarily in synch, so 'effective' is a nebulous term.

In terms of lubricating ability, we have seen reports (linked above) that oils actually lubricate better after thousands of miles than when they are new. That means less friction. For mechanical wear, that is a good thing. In terms of just the lubricating aspect of transmission fluid, I think the SAE papers are relevant.

In terms of heat conducting ability, I don't think there is a significant change. And in terms of compressibility (ability to serve as a hydraulic fluid), ditto. So for two of the most critical requirements of an automatic transmission fluid, the fluid doesn't degrade over time.

In terms of viscosity, I would expect the fluid to get thicker over time due to dissolved clutch material. The transmission has a built-in adaptation to that process with a feedback signal, so the fluid degradation in terms of higher viscosity doesn't matter until it goes beyond the transmission's ability to adapt.

Now to filters. A filter that is partially blocked will tend to filter better in terms of particles stopped (because the passageways are smaller). There will be a correspondingly higher pressure drop, and potentially a reduced flow. The pressure drop doesn't matter until the drop exceeds the ability of the pressure regulation circuit to compensate. But the filtering efficiency (amount filtered per unit volume) will rise. Until the filter can not flow sufficiently. Then it needs replacing.

My point is that a new fluid and filter isn't necessarily the best. They don't necessarily start out perfect and then degrade over their life. They can be designed to be optimum at mid-life, or at end of life (immediately prior to failure), depending on the characteristic involved. It is the same with many mechanical components.

Interesting debate.

And who says don't change the transmission fluid? That is a very broad question. I don't. BMW service manual reportedly does, up to 100,000 km if a technician has to drop the pan for another reason. Instructions I read were to reuse the old fluid. I say don't change the fluid if it is working fine, but it is the first thing to try (after checking the level) if the transmission is not shifting correctly. And I agree with your approach in that if you are going to change it, I think it is better to do it at shorter intervals than wait until 100,000 miles. That doesn't consider the value of changing it, just that it is a good way to manage the risk of higher mileage changes.

If we look at the ZF technical manual, and the info that RRPhil provided, the interpretation is slightly different.

Original post here: http://www.xoutpost.com/962737-post43.html

So, 10 years for normal use. Sooner for severe service. I'd put J. Belknap's supercharged 4.6 in that category, myself.

I believe that JCL is on the correct path here. When I had a faulty wiring harness in the valve body that was causing shifting problems (not unlike a faulty MAF) the dealership per the TSB refilled the transmission with the fluid that came out of it. It was explained to me by the shop foreman that if all new fluid was introduced to the transmission it would shock it (the transmission that is) and could cause dislodging of sludge, debris, etc that would cause problems for the transmission.

If you do drop & drains at certain intervals, your likelihood of having issues decreases.

Thanks for this....although i'm scratching my head for why my transmission shop used 16 liters on my 4.4i versus the 10 liters claimed by this note from ZF .. :dunno:

Did they flush the torque converter? That would require additional fluid.

11 Litre is for a completely empty system. A completely empty system is when you replace the TC with a new or rebuilt TC. Sounds like the shop is padding the bill.

Yes lncluding TC, maybe 16 including some detergent/cleaner ...i'll check again

First, I'm slightly confused why everyone thinks the transmission is a closed system. There is most definitely a vent pipe which allows for expansion/contraction of the air/fluids as the tranny heats/cools. If it didn't have this, every seal would start leaking very early in its life. The vent is higher up in the engine compartment and shaped to keep water out, but moisture and dust can get in there over time.

Yes, the filter does perform filtering function better over time becoming a super awesome filter, right before it becomes a perfect filter (100% filtering) at which point your transmission stops working because of lack of fluid flow. And yes, this will probably not happen until over 200k miles. But, lack of any maintenance before this happens, means that a fluid/filter change this late in life will result in all those varnish/deposits getting cleaned up all at once by the new fluid, and now you have clogged passages and valves, etc. So, new transmission or a complete rebuild (which is cheaper?) will soon follow the clogged filter.
Same story applies for the viscosity increasing over time. Everything has to work harder, which is accounted for by the adaptations, until the viscosity can't be accounted for anymore at which point you start having shifting issues. This should be at higher mileage again, and change the fluid, it will shift better again, until the the new fluid frees up the varnish/deposits and you need a new transmission or rebuild.

But, since there aren't many X5's out there over 300k miles that have regular 60k transmission servicing, JCL is correct that nobody knows what the actual life of a well maintained transmission will be. So maybe you spend $2000 in maintenance to get your transmission to last 300k miles. at which point it needs to be replaced or rebuilt. If you would have replaced the transmission at 200k without any servicing, you haven't really saved any money. And $1200 (3 fluid changes, 180k) over several years are lost earning potential.

Now, if it can be proven that the fluid changes double the life expectancy of the transmission, it will totally be worth it. (neglecting the lost earning potential of the maintenance money)
But, we need some guinea pigs to prove this case. And ideally, more than 100 vehicles doing regular maintenance to get some basic stats. Plus we need to find another 100 vehicles without doing any maintenance. And we need all 200+ vehicles to drive 400k miles.

But, if you drive your car hard as in lots of redline shifts, heavy towing, etc, you definitely should look at extra maintenance beyond typical mfr. recommended. Everyone else, probably doesn't matter.

obviously there are too many variables to do a consistent test. In my mind you would have


GM/ZF

Miles

change interval (if any)

Fluid choice

Driving style

Filter/no filter

enviroment.



would be tough to get a one size answer for these.

maybe include some people from the 5 and 7 series, although both are lighter vehicles and RWD only, so not that great for comparison with the X?
I thought about environment also, but even that has more variables beyond just weather and seasons like parking in a HVAC garage, carport/No-HVAC- detached garage, inclined driveway, or street at home, parking at work in a HVAC garage, no HVAC garage, parking lot, street. Or salt or sand for winter road maintenance? What types of roads they drive on and how often those roads are cleaned...

I just realized I never gave my opinion, just a bunch of interesting points that didn't lead anywhere.
Having a slightly different transmission in the 4.6, and the fact that the transmission was replaced at 60k, and had the fluid changed at 110k, I plan on changing the fluid again at 160k. But, if I was on the original transmission, at my mileage or any mileage above 100k, I would avoid changing the fluid and just wait for it to die of natural causes.