two schools of thought.
"life time atf or manual tranny fill from bmw is soo good that it needs no replacing". probably almost true. in both you get metal shavings after a time (30km? 60 km??), and heat destroys any oil after a while, so does oxidation. so it aint really lifetime.
1. replace regularly, every 30kmiles or so. probably wise, and again, the oil has to be correct for the tranny (that fosamax castrol or whatver the current fill etc). redline or other manufacturer will likely do fine if matched appropriately, but if you are not 100% sure do NOT do it, and just buy whatever bmw puts in there, like the prrior post recommends.
2. don't replace. in an automatic tranny: if you have not changed for 60km, don't start. all kinds of crap (shavings, debris, etc) will be dislodged, and go into clutch packs and destroy your transmission. regardelss of your oil choice.
3. manual can be refilled after a period of neglect, cos it drains much better, has no filter etc. but: if you use the wrong oil, you can destroy it too.
i destroyed first gear (some kind of different brass) synchro this way. took about 10 kilomiles for it to fail.