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Originally Posted by sunny5280
I suspect your shop saw only a small number of BMWs making the sample size too small to be of value.
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A very small number. But that is irrelevant, at least until BMW start to make their own automatic transmissions.
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Originally Posted by sunny5280
Would you mind posting the supporting data?
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Yes.
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Originally Posted by sunny5280
I don't know if it does or not. We replace every other fluid in our vehicles in order to increase service life so I fail to see why the transmission fluid should be any different.
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That is getting to the essence of the debate. Think of the risk of an fluid-change induced transmision failure as x. Think of the benefits of clean fluid making the transmission last longer, and that being relevant to the life of the vehicle, as y. It is simply a case of whether the risk of x is greater than the benefit of y. X is greater than 0, it isn't 0. It isn`t an urban legend as you described it above. Y used to be quite large, which is why we used to change transmission fluid every 30,000 miles or so. Y isn't so large today, because we now have factory installed transmission coolers with thermostats, we have lock-up torque converters that keep the fluid temperature down, we have programmed shifts that reduce clutch slip, and we have feedback loops in the actuators so the transmission can compensate for fluid viscosity change over time. Presumably clean fluid could make it last longer, but there are posters here who are over 200,000 miles on the original transmission fluid. Many transmission will last longer than the chassis, simply because it will become too expensive to do software updates. These are not cars that are going to be cheap to own as they age, and that is going to greatly reduce the number of them on the road. At the same time, there are lots of early hour transmission failures (lots is relative, let's just say more than we would like to see) and they do not appear to be related to oil quality, but rather random failures of software, sensors, etc. Not things that would fail less frequently if the oil was cleaner. What happens is that because of those early hour failures, owners consider changing the fluid to improve their odds. That is faulty logic.
It is simply a judgement call that each owner has to make. There is no right and wrong answer, there are just risks. I think it is worth pointing out that not changing the fluid is not some nonsense dreamed up by a marketing organization at BMW, there is a technical reason and in many cases it makes sense. If owners are going to change the fluid, then I think that they have to change the filter at the same time, they probably should change it two or three times in short order to get the bulk of the old oil out (since there is no torque converter drain), they should use factory spec oil and not try the roulette game of various aftermarket oils that may or may not work, and they should refill it according to the BMW procedure that involves taking fluid temperature readings during setting the level.