I just put a load of 0W-30 into one of my outdoor cars, a 2004 Dodge Intrepid 3.5L for one reason: cold starts. Yesterday, I took advantage of Costco's $10 coupon on cases of 5W-30 and picked up three of those. It got me thinking about just how much different the specs on these two oils are. I guess the answer is, "not much", but I found some unexpected differences. For example, while 0W has a lower viscosity at 100C (where you'd expect them to be the same), 0W has a higher viscosity at 40C, 62.9 vs. 61.7. Does that trend continue at even lower temperatures? The Viscosity Index of 5W is higher than 0W, 172 vs 166. I thought higher VI's meant better cold starts. Am I mistaken? The Pour Point seems to indicate what one would expect, with 0W lower at -50C vs -42C. Mobil gives an MRV @-40C for 0W but not 5W, so that can't be compared. Not related, but of interest, Mobil specs a seemingly healthy TBN of 9.1 for 0W then doesn't provide the spec for 5W. I'm sure I could use either oil in any of my vehicles and I like the fact that the 5W is Honda HTO approved when considering it for my turbo Mazda but the real issue for me here is cold starting. What say you oil spec experts about the differences between these two oils? Heck, the Mobil website even gives the 0W a couple of extra check marks on performance. Have UOA's revealed any significant differences? Is it really worth bothering with the AFE's for near 0F temps?