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Old 03-27-2011, 09:46 PM
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Yorgi Yorgi is offline
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Location: Toronto
Join Date: Mar 2005
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Mein Auto: '04 M3 / '06 650i Cab
Lifetime transmission fluid is one of those "black magic" subjects. Very difficult to get a straight answer since it is hard to prove the different trains of thought.

My understanding of this issue is the same as the OP. Changing tranny oil on a car with high mileage can result in accelerated transmission failure. A couple of BMW techs have also told me this. The techs told me that the clutch packs in auto transmissions have friction material on each disc. As the friction material wears off, it is suspended in the oil. Oil with suspended friction material helps the clutch packs continue to work even though the discs now have less friction material on them.

Change to new "slippery" oil and the worn clutch packs will wear out at an accelerated rate compared to just leaving in the old oil.

The techs also said you still should change out the oil at 100K miles because by that point the fluid is contaminated to where it does more harm than good.

There is no way I can verify this info but it sounds reasonable to me. Luckily I have a 6 MT so I don't have to worry about the "to change or not to change" dilemma.

Don't worry about oil shortages, we've got you covered here in Canada with the tar sands.
Straight lines are for fast cars...turns are for fast drivers
'04 M3 - track
'06 650i Cab - daily driver
'02 Highlander - kid hauler
'01 330Ci - winter beater

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