Other than buying the 0.5 ml bottles of the Castrol SAF-XJ 75W-140 from BMW, it is almost impossible to find this gear oil in the US. I located what I believe to be the same exact gear oil produced by Castrol for use in the Nissan R35 GT-R. It is available from the Nismo Shop in California. It cost $43 per 1 litre can which is about 50% less than BMW pricing (per volume). Unfortunately, since the differential takes a little over 1 litre to change, you end up with surplus when you order 2 cans. If you team up with a buddy, you could order 3 cans and split the cost. Also since it comes in cans, versus the BMW bottles (with tubes), you have to transfer to an appropriate dispensing bottle.
http://www.thenismoshop.com/Castrol-...T-R-_p_48.html