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JCL · #**107** 12-03-2013, 04:42 AM

Well, I think we are starting to just repeat previous posts, but here goes.

If I had a heavily modified X5 with more power being put down than BMW ever designed the vehicle to handle, I would say all bets are off. Normal maintenance rules don't apply. Neither is the experience gained with that vehicle appropriate to apply to stock vehicles. You are outside the design parameters. Do whatever feels good. Changing the fluid may make it hang together longer. I have no idea. It is a sample of one, and that doesn't tell us much.

I don't agree that aged transmission fluid by definition does not perform as well as new fluid. It can certainly do so, if it hasn't been overheated or contaminated. It doesn't wear out unless it is overheated. It will change over time, agreed, primarily in the amount of additives that remain. That is to be expected.

I think you are suggesting that if the fluid is less than new, then the transmission won't be working as well as it should be. I don't agree. The transmission was designed from the start to deal with a fluid that ages and changes properties. The feedback loop for clutch engagement times (measuring frictional characteristics) is an example. The transmission can also deal with widely varying fluid viscosities. Those features were designed in so that the fluid could age and not impact transmission performance. It is why the transmission has adaptations stored; it is adapting to fluid condition and its own performance.

The diesel lube article was in response to numerous claims in this thread that all fluids wear out, and that new fluid is always better. That turns out to be a false claim. Yes, I used data about engine oil, but it seems only fair, since some posters continue to use engine oil replacement as a proxy for transmission fluid changes, and talk about making the mechanism last longer from fewer metal particles in the fluid. I think that if we want to justify a transmission fluid change we need more than "because it is right to do so". At the same time, I think that people should do what they want to do with their own transmissions. I am not attempting to talk people out of changing their own transmission fluid if that is what they want to do. I am saying that they should recognize that justifying their strategy with "because it is obvious" and simultaneously putting down others who don't share that strategy isn't logical. I don't like reading attacks such as "what happened to pride of ownership" and "if you cared about your vehicle you would..."

A look at when components fail can tell us something about the common failure modes. If the failures are all grouped in a bell curve, then wear is worth looking at, because wear should relate to use (miles, hours, number of shifts, whatever). If the failures are all over the place, from 30,000 to 250,000 miles as reported here, then logically something else is going on other than straightforward wear. There are maintenance management strategies that study that phenomenon. I attached one. All I am saying is that metal wear is not likely to be the primary cause of transmission failure. I know that many like to say that these transmissions tend to fail at 100,000 miles, but I don't agree with that conclusion. I think they fail at all sorts of mileages. And the random nature of the miles to failure in itself contains a clue to the cause. I think it is complexity. It is the famous o ring seal. The actuator. The snap ring. There are a myriad number of failures. And they occur in transmissions from two different non-BMW manufacturers, in three different X platforms that I follow.

We don't have statistical data, agreed. We have frequent anecdotal reports of transmission failures. But do we have reports of failures caused by fluid degradation? The first sign would be burnt fluid. Not much reference to that in the almost nine years this board has been running? Any burnt transmission fluid, when not caused by another failure, like an actuator? Any apparent higher incidences of failures when towing trailers up to 8300 lbs? We even have people here with nitrous and superchargers. Are transmissions failing at a higher rate there? If the failures were mechanical wear, increased load would cause earlier failures. And if the fluid was wearing out at 50,000 or 100,000 miles, it should be pretty much impossible to get to 250,000 miles. Just as a reality check, we haven't seen many reports of engines needing rings and main bearings either, despite the fact that most X5s follow factory maintenance recommendations. We just aren't seeing mechanical wear dominate the failure reports as we used to decades ago. That is what should cause us to challenge previously held positions about what are appropriate maintenance strategies.

Since the thread was titled "Thoughts on changing transmission fluid" I figured it was an open discussion. I didn't think there was supposed to be a winner. There certainly isn't a single right answer. But there can be some new food for thought.

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