Castrol doesn't make a 10w60 unless it has just come out. Castrol DOES make the BMW labelled 10w60 and is available outside the dealer but only in Europe.
You should be using synthetic anyway. The M5 gets 10w60 if it has a build date of 3/00 or earlier. Build dates after 3/00 can use 10w60 or 5w30.