Hello bast313. Motor oil in the US is much cheaper than in Europe. Some of the BMW dealers here are charging about $5 or less per quart for the BMW/Castrol 5W-30 which is I believe equivalent to Castrol Softec in Europe. The Castrol synthetic that is readily available in stores here in the US for about $5 to $6 per quart is an ACEA A1 oil that is not longlife rated, thus it is worthwhile for us in the US to buy the oil from the BMW dealer.
In the big cities, Mobil 1 0W-40 is readily available for about $7 per quart.
Check out the used oil analysis forums at www.bobistheoilguy.com
and you will see that the BMW-Castrol oil performs well as long as you keep the oil change interval reasonable (one year and 16k to 18k miles is not reasonable in my opinion). Of course, Mobil 1 0W-40 also performs very well.
addendum: A downside of being here is that we don't have the same variety of availabilty of autos, especially diesels like you own.