My first was at 1500 miles. Gray and gooey.
Second was about 12,000 miles further and Blackstone tagged it with too much stuff (iron and chromium). Wheel doesn't have any play...yet. It too was also gray and gooey with a bit of fuzz on the magnetic plug that I installed (BMW didn't install this on first '06's). They recommended no more than 7,000 miles before the next change and submission. I may do it sooner 2,000-3,000 miles just to see what comes out. Hopefully not as bad - maybe?
I suspect Robert is right: 6-7k miles after the first 600 would be a good idea. If my memory is correct, I believe BMW never did clarify what if any changes were to be made after the 600 miler?
I might even suggest a first 300 miler if possible just to flush any machining crap out of it. BMW had some additives (moly?) in the mix that gave it that gray color. I wouldn't be too partial to the suck-it-out-via-a-tube method for fear of residual material being left behind.
Those guys whose final drive oil comes out straw or red-colored (BMW/Spectrol) really bug me!
Fwiw, on some forum (ibmr?) there was a mileage poll of when they fail. If the drive makes it beyond 60,000 miles it may never fail. My '96 RT ran up to 100K and never was replaced. Come to think of it, it's oil always came out pretty clear too (Mobile 1 gear oil). I changed it every 6,000 miles back then and I often thought it was too much as it was always so clear. It had a drain plug too.
Anyway, a lot of final's in the poll crapped out at 8-15K, and another group - and some subsequent - goes out around 33K. Chances are 1 in 6 that it will fail and it seems to be BMW's Achilles's Heel - not great odds, but BMW seems to live with that. And I believe 4 drives died in the 2007 IBR too. I believe your chances of it not starting at all are probably much greater (e.g. battery, switches, electrical gremlins).