Themm!!! this is really freaking me out to perform because I don't want to mess-up my transmission worst by adding more or less oil fluid there. Once the system is such complicated like that, where you will never be able to drain out all the dirty oil that contained inside the (Transmission system) I rather leave it as it is, instead of going through all these procedure.. I wonder why BMW stated (Lifetime warranty for this type oil.
It should be easier as changing the entire engine oil at once.