After doing a search on Delo 400 in gas rigs, I've been convinced to change. Truck has 116k on it. Been thinking of going to it for a while. My question is of the 15w. I live in Denver, Colorado, our winters can get cold. Could in the fall I change back to 10w then in the spring go back to 15w40? How big of deal is it to have 15w50 in 20degree weather? thansk.