Maybe I am old fashioned, but it makes we nervous to use a 5W30 oil in Southern Colorado and the Southwest when temperatures in the summertime reach 100 or more (easily 120 in Arizona and Nevada). I like to use a 10W30 in the summer, or the thickest oil allowed in the owner's manual. I have a Saturn SL2 and the thickest oil allowed is 10W30, but 5W30 is preferred. I know a woman who bought a new Honda Civic and the preferred oil is 5W20. That just seems too thin for me unless it is synthetic. What do you guys think? Also, what do you guys think about using a 10W30 Mobil 1 in the wintertime? Will it still flow as good as a 5W30 conventional oil? A friend of mine used to work as a professional mechanic and he told me he uses Mystik oil. My Dad (when he was alive) liked Mystik also as well as some other men who were mechanics or involved in oil distribution.How does Mystik compare with Castrol, Chevron, and other oils that you guys have tested?