Q about cold and oil

[doorman]

A very warm thank you to all for making me feel so welcome!

Does anyone live in extreme cold areas? I've seen some pics lately of snow on BMW's. I reckon that qualifies.
I've read quite a few posts and articles about oil comparisons, synthetic and petroleum based. I've already made the decision to use syn. Most likely Mobil1. I'm even going to switch my diesels to syn 15w-40. I'm quite confused about what weight I should use in the BMW. The dealer says use 15w-40. They are in Sante Fe. Not quite as cold down that way. In the harshest part of the winter here it will get -35F quite a few times. Thats the low extreme. The usual, like right now, gets down around -5F or so. It will warm to 25 maybe even 40 some days. I plug in all the diesels so they'll start.
Suggestions please?
Is Mobil1 ok? Royal Purple?
Any other brands recomended?
Any info would be greatly appreciated.
Sam

[632 Regal]

M1 0w-40 is "all weather" for all extreames.