Auto trannies tend to heat up a lot more, so thinner fluid may not be a great idea.

Some people use the redline ATF but the actual spec for manual transmissions is either bmw MTF LT1 or LT2, depending on the year. But that is not really easy to acquire, and isn't cheap. I used RP MTF in mine.

I believe RP synchromesh is an acceptable substitute for the ATF, but there is a lot of conjecture and theory on this all over the internet, as far as the MTF goes.

I would not be so quick to assume acceptability. Some transmissions do spec ATF, but some spec 10w30 and still others call for other things. Heat is the biggest factor in automatics, and it is not nearly as big of an issue in manuals, where thinner lubricants would be acceptable.

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2001 E53 3.0 5pd