I agree that eventually, the contaminants in the fluid will cause problems with the transmission. I'm just not convinced this will on average happen before other components not related to the fluid, such as the clutch packs, wear out.

I'm not saying it's a bad idea to replace the transmission fluid, and in some cases, it might increase the life of your transmission. But I'm with JCL that in most cases, it won't really do much, if anything. It definitely didn't in the case of my daughter's E46. Otherwise, as I said, there would be thousands of cars in this country with dead transmissions, when in fact, transmission failure really isn't that common of a problem.

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2003 X5 4.4i
1992 Porsche 968
2007 328i
2014 Subaru Forester XT