Flash1034 posted it and this was the SRT Engineers response:
This question has come up a few times on this forum with some misinformation being passed along - Just for the record; 15W-50 will work fine in the 2008 engine. The only downside might be a slight drop in fuel economy (0.2 to 0.5 mpg if you can measure it or if you care!). The 0W-40 oil was selected primarily for the slight fuel economy improvement it offers and its broader operating range when compared to the older 5W-30 or 10W-30 oils. The 0W-40 is a superior product when compared to the 5W or 10W-30 oils.
The only caution is as Mark stated, make sure the 15W-50 is up to operating temperature in cool or cold climates before you run any moderate or high engine speeds (good advice in any case!). In some instances, the 15W-50 can actually improve cam control in new engines when the cam and valvetrain friction is high (for example use 15W-50 for the first oil change and the 0W-40 thereafter). The higher viscosity and shear strength improves hot oil pressure and provides more control force for the VVT system as a result. Certainly use the 15W-50 if you are on track often or in high temperature climates. Hope this clears up this question!