If we're to believe everything you've said about transmission fluid I'd have to conclude they changed their minds.

"Changing their minds" is an old wives tale. Or an urban myth, take your pick.

Your premise, burden of proof over to you ;)

I am not BMW, so I say for sure. But I can tell you how a typical design engineer would define it when specifying a fluid change interval.

It is not a person's lifetime, because there is no instruction to change it in the event of untimely death of the vehicle owner.

It is not vehicle lifetime, because there is no instruction to retain the old fluid and reuse it when changing a failed transmission out with a remanufactured transmission.

It is obviously transmission lifetime. However long that transmission may last. And a standard, frequent reason for not recommending changing a fluid during the component life (which is variable) is because of a lack of benefit of doing so in terms of extending the useful life of the component. That may or may not include negative impacts of the change procedure, but definitely includes a lack of positive effects from changing it.

Everything else approaches a conspiracy theory.

What a stupid response.

Has that ever been in dispute? Regardless that does nothing to define what lifetime is.

I'm trying to believe you, but having difficulty getting over the statement "Old fluid lubricates fine."

Part of the reason is my experience is similar to Lamby's:

On both the E320 and the X5, the shifts seems noticeably smoother after fresh trans fluid was installed.

I've read both sides of the argument, and I'm concluding that driving conditions may be the most important factor. For us (lots of hills w stop & go) it makes sense to change it, while someone who drives mostly highway and on mostly flat terrain might be just fine without a change.

You do understand that it is your previously (and frequently) posted position, right? That the one putting forth a premise has the burden of proof on them? And you certainly know where I got old wive's tales and urban myth from. Welcome to August 2013.

The first point is, that an oil never stops being slippery. The lubrication aspect, keeping the metal parts sliding, doesn't change. The metal particles that some measure with oil sampling are microscopic and don't impact lubrication, unless they are doing large particle counts. Particles that are large enough to impact lubrication are generally filtered out, until the filter is clogged. What is held in suspension is benign. Viscosity can change, sure, but modern transmissions compensate for that via electronic controls, up to a point.

Yet many who change fluid early do so to try and extend the life of their transmissions. Fair enough, but if you take the lubrication demand out there is less justification there.

What does change in an old transmission fluid is the amount of additives. Anti-foaming, anti-oxidation, and so on. If your fluid is foaming, or is oxidized, then shifting may not be smooth. But that doesn't wear out your transmission, it isn't a lubrication issue. The most important additives relate to friction modifiers that impact wet clutch engagement. These additives not being present is a frequent cause of poor shifting. See what happens when you put the wrong spec fluid in. The difference is due to these specific additives.

So if a transmission isn't shifting correctly, it is reasonable to change the fluid to see if that helps. The fluid may be out of spec. But that doesn't mean that a transmission that shifts fine will benefit from a fluid change. The paradigm that it is an oil just like differential gear oil or engine oil doesn't apply. The former has hypoid gears and high pressure; the latter usually breaks down from acids which are byproducts of combustion.

Why don't you add to the discussion by giving your opinion of what "lifetime" is instead of just barking and howling at JCL all the time? It's tiresome at best and not in the spirit of Xoutpost. :rolleyes: