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Originally Posted by John Galt
GM uses the exact same transmission as the early E53's that supposedly have lifetime fluid. GM suggests changing the tranny fluid (with DexIII) every 50k miles under the following conditions:
If the manufacturer of the transmission suggests 50k mile fluid changes under moderately heavy use, I think I trust that more than I trust BMW's "lifetime no matter what" approach.
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I see your logic.
However, the criteria listed by GM for extreme service are related to both heat, and clutch pack wear. It is a matter of the safety factors design in to the system. The transmission fluid heat will be impacted by the trans cooler sizing and capacity. The clutch wear is directly impacted by the shift logic, and the firmness programmed into the transmission control module. It is also impacted by the BMW electronics that back off on engine power when the shift is executed, thereby reducing shock to the clutch packs. My point is that all of the above are different than the GM application. I don't think that GM knows more about BMWs than BMW, and I think that the BMW lifetime fluid recommendation is a joint engineering decision based on a multitude of factors. But I could be completely wrong, and the guys who printed the manuals could have decided it without talking to the engineers.
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