Group III OE - 7500 mi -12,000 mi oil
Group III XL - 10,000 mi - 15,000 mi oil
Group IV Signature - Up to 25,000 mi in the correct application.
OE is built for typical OEM Drain Intervals.
XL has a more robust additive package for extended service.
Signature is the best oil they know how to build short of becoming to expensive to be commercially successful.
If you are going to do 6 month - 5,000 mi OCI in a typical passenger car you are unlikely to see a real difference.
Base oil blends are anyone's guess these days but my guess would be what Gene K has said. SS is simply the best oil they can make at that price point. Based on FTIRs, SS is a PAO/Ester blend. Decent amount of V too. (estimated range 8-13%.)