If you are in warranty ask your servicing shop, and don't argue with them.
Mine knows I want to use Synth and told me we will look at it at the 12,000.
I trust them so I will go along with them as I have said before, in a warranty situation, they are your ONLY advocate with BMW. I am don't want to be hanging out for anything by not following my Service Managers advice.
I ran Nikasil barrels (sp) on my Harley and at 2000 miles I had Synth in it. I also heat cycled the motor and seated the rings pretty aggressively. At 2000 miles compression was right where it should be for the cam, heads, and deck height. Leak down was within 2 of each jug.
So Brian is correct that as soon as rings are seated you can run Synth.
Now flavors of Synth will just cause everyones PMS to flair but I like Amsoil, or Mobil 1. I have used Royal Purple, Red Line also.
I was using a Synth 75-90 in the FD, but I will go to 80-90 after reading a bit more on it, but have huge doubts that it matters one bit between the two.
In the trany I use 90-140 Synth.
after the 600 I changed at 3000, and will change it at 9000 also. I used good old Castrol GTX 10-40 because it was Fall here in PA. I am due for my 6000 and will get BMW oil from my shop. At 9000 I will do it again and use 20-50 Castrol GTX as it will be summer. Good Dino oil is cheap and 3000 miles on it is enough by my opinion.
The cool thing is my shop don't care if I carry my own oil in because of what I prefer. So I take a quart of each Synth for the FD and tranny and they don't mind a bit. When he says I can go Synth I will carry four quarts of that also.
He gets labor, I buy the filter and crush washers for my in between changes from him, and usually find something else I want, or need to buy to support him.
Just the way I look at maintenance, and my shop.
Sorry so long