To answer your question about alternating oil between RP and Amsoil, I believe the Amsoil is a better product than RP, so why keep going back and forth to an inferior oil. Secondly someone else is trying to push the Pennzoil because that's who Dodge Chrysler has a partnership with. My opinion is that Pennzoil also is not a better oil than the Amsoil. Northern Sask. winters are brutal, I imagine even the summers are short and cool. I would stay with Amsoil SS 0w20 or 0w30 if you're more comfortable with a 30 weight. JMO. Good luck.