Hmm, I think the 10w-30's are more stable from what I know. My truck recommends a5w-30 all year but a 10w is exceptable. Myabe I'll run just a 10w year around. Colorado gets cold but not cold enough in my opinion to mandate a 5w or lower. My truck works hard and I just want something to stay in gade. My wifes 03 Hyundai Sonata LX V6 will run anything from a 5w-30 to a 20w-50!