I just had to install this stuff in another M3 today for the noise problem. I gave the bottles a thorough inspection, and there is no mention of Castrol anywhere. It does say "produced in Germany for BMW AG". It appears Castrol has no involvement in producing this product (other than making the base oil). So the final word on this discussion is this:
The "special rear axle fluid" starts out life as the regular Castrol SAF-XJ synthetic oil. The SAF-XJ oil is what comes in the car from the factory and is what is used during the 1200 mile run-in service. BMW then modifies the SAF-XJ by adding some type of additional friction modifiers or "FM booster". This oil is sold under BMW part# 83 22 2 282 583 and is only used when a rear differential noise concern is encountered.
You could probably obtain the same results by adding a tube of friction modifier to the existing oil, but for some reason BMW chose to go this route. Hope this clears it up for everyone.