I wouldn't worry about getting Castrol. There's better oil out there, although that's a whole different argument. I personally prefer Mobil 1 and have run that in all my bikes and cars with no problems. As long as you stick to the proper synthetic weight you should be set. Don't let BMW fool you that you can only use their own oil...
I ordered a 10 pack of filters from germanfilters.com and they shipped em very fast for a great price (about $40 shipped within 2-3 days). Both my 525i and Z4 take the same filter.