When the going gets tough, I think you'll be better off with the 5W-20 vs. the 5W-30. Why?
Conventional oils are formulated using the lower of the viscosity numbers on the oil bottle. A conventional 5W-30, for example, is formulated with 5W oil, and Viscosity Index Improvers (or VIIs) are added to bring the oil up to 30W viscosity under normal operation temperature (which is in the 10 cSt range at 100*F, give or take). During normal use, oil will shear, which breaks it down. A conventional oil will actually lose some of its viscosity, and will try to retrograde back to its native viscosity (in this case, 5W). The oil will "thin" out, and won't really be a 30W oil at normal operating temperatures. It's often said (when folks argue against 5W-20, and I'm not saying that you are) that a normal 5W-30 or 10W-30 shears down to close to a 20W oil after a few thousand miles anyway. Because the 5W-20 oils don't have as many VIIs (because they're only going from 5W to 20W, instead of 5W to 30W), they don't shear down as much, and stay in grade better.
Synthetic oils, as I understand, are formulated using the higher weight oil (20W or 30W) and use Pour Point Improvers (PIIs) to improve the cold pouring point. In this way, there's no where for a synthetic oil to "shear to", since its native viscosity is already what you want to be running anyway.
I think the real benefits of these new 5W-20 oils are demonstrated in Ford having retrospecified that oil for many of its older engines. The argument that the oil is only used for CAFE reasons goes out the window. Ford has nothing to gain by you running 5W-20 in a 1999 Crown Vic vs. 5W-30. And they have specifically EXCLUDED this grade of oil for SOME of its previous engines, indicating that it really is not compatible with some engines, for some reason. The only reason I wouldn't use 5W-20 in a pre-2005 3.3L/3.8L Chrysler engine is because Chrysler hasn't specifically retrospecified it yet. If they do, I'd have full confidence that the engine is 100% compatible with the oil. If they don't, then at least some part of me will feel that there truly is a key component or components in the 2005+ engines that allow for and are optimized for the 5W-20 oil vs. 5W-30. The owner's manuals used to allow 10W-30 vs. 5W-30 if you were in warmer weather, etc. Not anymore. The ONLY grade allowed is 5W-20, and I'm confident that there are valid engineering reasons for it.