This is a quote from the Adv forum...I never investigated the clean bearing and am wondering how true this is.
I found absolutely no grease and the only lubrication I saw was blue? what? oh wait, it was redline sp lightweight. The same stuff I run in my gearbox. So why put sealed bearings in if they don't keep their life-long grease in?"
I was under the assumption that the bearings were packed with a lifetime grease and sealed. Turns out the bearings are actually 'clean bearings' and the seal is really a filter. They are in fact lubricated by the gear oil. Clean bearings are purported to outlast open bearings significantly."
Rereading this, to clarify, this wasn't my answer. But one that was offered on the Adv forum. I am curious about this because finding dark oil drain out of the trans when changing fluid was normally thought to be that the grease was released from clean bearings.
I think the bearings are packed with a red or brown grease. And after a while some or all of the grease migrates out. I have personally seen two cases where fresh synthetic gear oil turned muddy brown within about 1,000 miles of it's installation. Then when changed again, didn't do it again. And I've been told of a few other cases of the same thing. I am convinced the grease was washed past the seals.
However, it appears that gear oil finds its way in because in both of these cases the transmissions continued to work fine for thousands of miles.
I would surmise that BMW accomplished its objective - protecting the transmission bearings during break-in when metal is sloshing around in the gear oil.