German Castrol is 0w30 (MADE IN GERMANY). All other Castrol (betsides 5w40 I think) is made in USA (group III)
German Castrol (Group IV?) is a highly rated oil that performs better than amsoil in some engines. Regular Castroil is a group III oil (hydrocracked/dino with boat loads of additives).
Difference between Green and Gold GC is that Green was the "old" formulation COLOR of the oil (open the cap and the oil is neon green). The new formulation (equal if not better to green) is Gold colored.
As for BMW OEM oil - My friend with a 330ci is going in for an oil change. I asked him to ask the mechanic for the used bottle. I'll let you guys know what I find. Friend won't go in for another 5,000 miles. (One of those: BMW engine can "handle" 30,000 miles on one oil change types).
I've got a bottle of the BMW oil right here in front of me. The only info that might be meaningful that I can see is that it is ACEA A3/B3 and API SJ/CF. I have had this bottle for probably a year, so things might have changed. It is a 5W-30.