I use the 0-40 synthetic from the co-op, its CI the guys use it up here in the logging trucks, so if its good enough for them, I have used it for just over 60 thou so far.

 

I lived in Winnipeg for 12 years and found that synthetic oil in the axles, transfer case and gearbox is also very important. Where I parked at work it was very open and remote, I used to park in low range just so I could get the 4X4 to move when there were extremely low temps and hi wind chills especially in the middle of the night.