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.