OK, let's say I took a sample of my transmission fluid. What exactly would I get analyzed in an oil lab that would answer this question? Not lubricity, as it is not part of a standard oil sampling test. I touch on this only because you mention friction based devices. Viscosity? It matters, but recall that the ZF transmission automatically adjusts for changing fluid viscosity over time. If you want to throw out the fluid based on a viscosity reading you need to know the acceptable limits for that adaptation. Water in the fluid, sure, that would be a good thing to know. But you could see that without a sample. Large particle analysis? OK, but that indicates your transmission has already failed. Not a good predictor. All of the wear elements that people like to focus on as indicating wear rates? OK, but those aren't changing the quality of your oil, as they are measured in parts per million. Those numbers are used to trend transmission wear rates over multiple samples, not to qualify whether an oil is any good or not. IMO, here is the one that really matters: the effectiveness of the remaining friction modifier additives which affect clutch engagement. But how would you test for that? It is when those are used up that you can get flare ups on shifts. It is just that there isn't a test to say that a fluid still meets the original spec, unless you run it through the full ZF lifeguard certification process.

I understand fluid analysis. I even got a chance to work in an scheduled oil sampling lab at one point. I ran tests on fluids. I just don't know what a lab would do with a sample of used transmission fluid that would quantify how good it was, beyond the visual inspection that you can do without paying a lab (burnt smell, water in the fluid, etc)

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2007 X3 3.0si, 6 MT, Premium, White

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