The Mobil 1 0-40 has a pour point of -54c so with that news there is no point going away from the Castrol 10w-60 unless you live in Alert!
http://www.imperialoil.ca/Canada-Eng...il_1_0W-40.pdf
The 5w-50 has a pour point of -51c which is the same as the Castrol 10w-60.
http://www.imperialoil.ca/Canada-Eng...il_1_5W-50.pdf
Yeah maybe BMW does know best in this case.