I have 3 friends that use both pennzoil and castrol gtx all winter long. It can get **** cold outside here. The question is, are they killing there engines faster by using these oils in the dead of canadian winters?
If the temperature is consistently getting down to -35C in your area, you need to convince your friends to start plugging in their block heaters overnight, or switching to synthetic 5w30 or 0w30. I for sure would not want to be running a dino 10w30 in those kinds of temps. They are most certainly reducing their engine life significantly with each -35 degree start they make (assuming their car is parked outside overnight, if it's inside the garage it's a different story)