You could have one 5w oil show better cold weather specs than another though. Look at the cold cranking performance to see. One 5w oil might just barely qualify, while the other could be considerably better than the specs needed to be a 5w.
A 5W-20 and 5W-30 motor oil must meet the same cold cranking viscosity limit at -30 C. An oil can be thinner than the CCS limit but not thicker. I do think a 5W-20 oil should be significantly thinner at any temperature over about 0 C. Most of the time when you are starting a cold vehicle it is not near the -30 C temp for the cold cranking specifications. The Ford WSS-M2C153-H specification goes far beyond the API SL and ILSAC GF-3 requirements by requiring a double length Sequence IIIF test with a lower limit for viscosity increase. The limit for the TEOST deposit test is also lower. These specs are not easy to meet and require a better base oil or more additives or both. At least some of the Ford approved 5W-20 motor oils also have a significant amount of synthetic base oil. For example the Pennzoil 5W-20 is about 70% group III and 30% group II+ (not counting additives). The regular 5W-30 is made-up of group II+ as far as I know. My point is that many, but not all, 5W-20 oils will actually have a lower cold cranking viscosity than the same brand of 5W-30 motor oil as well as clearly being thinner at higher temperatures (over about 0 C).
