I thought I should reply to ard's comments;
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What does "well served" actually mean?
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By that I mean to infer that the results of their (ZF's) recommendations do not create a negative perception of the company or its products.
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Do you mean that ZF believes the longest service life will be achieved by not ever changing the fluid?
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No, I did not say that and with the aforementioned 100k recommened change interval, evidently ZF does not either.
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Or has this become a calculus of 'ownership costs' and 'owner hassle' and 'likelihood of trade in' combined with odds of failure to determine this is 'best'?
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Isn't that suggesting a planned or expected failure rate? I cannot answer that but certainly don't subscribe to it.
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I'd like to think ZF & BMW have ONLY the longevity of the AT assembly as their sole determinant as to the fluid service. Until that is confirmed, I thin the community is ill served by broad assertions that 'high tech' fluids will save the day.
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To me it's the sum of the parts (the lubricant being a part) that allows the recommendations to be made. It isn't about a "high tech" fluid, rather it's about the total system design. No broad assertion regarding fluid quality.
In regards to my suggestion for an oil sample valve you asked;
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Personally I suppose I agree, but I need to ask you "why?" Isn't the BMW interval of 100,000 miles satisfactory?
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m5james post suggests that driver actions are a contributing factor to transmission and other component life. He also states that he adheres to routine transmission fluid changes as a matter of what he (and you and others believe) to be effective in providing longer transmission life. And while I do in fact agree, the difference I would suggest is that by having an oil sample valve, one could in fact determine the appropriate drain interval for a specific transmission driven by a specifc owner in a specific environment.
We always want to change good fluids as opposed to changing fluids that are deteriorated which as m5james states
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there is absolutely NO fluid that doesn't degrade over time. Period.
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The challenge is knowing when to change the fluid to achieve the elusive maximum transmission life goal. Without analysis, fluids may be changed prematurely, or perhaps, (I can only surmise) just before they dramatically degrade, assuming the 100k recommended change interval is in fact based on engineering or empirical evidence which suggests it is the most appropriate interval for the majority of applications.
And thanks to JCL for the Maintenance Management document.
Great thread!