From what I gather you have a stick. I would not replace that fluid, not would I replace differential fluid. That gear oil is thick and i do believe the MT and Differential oil is lifetime. Do some research. I had an E46 that i attempted to do this on. I used Redline as well. When i drained the gear oil at 133K it was still good. Therefore I put it back in and saved my $. After some research I found that many others experienced the same but went thru with the change and the Redline broke down faster than the OEM. But you have to make your own choice. Me, I wouldn't do it.

 

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From my experience in other makes, the MT fluid is essentially auto tranny fluid, i.e., not that thick. Yes, the rear diff fluid is usually 90W or something (yes, heavy).

Have you ever been the BavAuto.com? They have some recommendations on maintenance cycles.