Indyxtype

The first number simply denotes how well the oil flows and gets into tight places when it is cold. In the spring/summer when this "cold" temperature is already reasonably warm, your 5w is fine. Come fall/winter when the temperatures start to drop, especially if you're in a cold climate, it might be worth your while to locate some good 0w. To give some significance beyond just numbers, take a closer look at your oil the next time you do an oil change and note just how much thinner the warm oil coming out of the pan is then the cold stuff you are pouring in (you are warming your engine oil before you drain it right?) Or if you really want to be a weirdo and get scientific, I suppose you could put some oil in the freezer, then pour it into a container and slowly heat it on the stove or something and watch how it changes over the temperature range. Its a little scary just how thick - 32 degree oil is which is why I use the thinnest stuff I can find in the winter.