Mobil claims the AFE can save you fuel...YMMV. iirc, they are comparing the 10w-30 vs. 0w-30 in the EPA city test. This test is good for a 0w oil because of the cold start conditions.
If you are using a 10w conventional now AND switch to a 0w synthetic AND are in a cold environment AND take short trips AND have a healthy car, you will see a reduction in fuel consumption.
The biggest adjustment you can make to improve your fuel economy is adjust the nut behind the wheel!