Well as far as the fuel mileage argument goes the lower weight oils such as 5w-20 are more fuel efficient than say a 10w-40, because the lower number mean the oil is overall a 'thinner' more free flowing oil, therefore giving less resistance. I've heard of maybe 1-3 mpg differences by changing from a heavier weight like 10w-40 to 5w-20, but I generally like to go with what the manual says for the car. So...in short it may help gas mileage but really not a whole lot. The reason for Ford's choice of 5W-20 would not be based on mileage.
By changing over to 5W-30, you are not necessarily increasing protection, because as some know the 'W' in the 5w-20 doesn't mean weight, but winter. Oils are rated by the SAE at 32 and 212 degrees and tested for their viscosity and flow at both temps. So depending on your application and environment, ie, if you live in the frozen tundras of Russia, instead of a 5W, I would use a 10 or 15W for the extreme cold. The second number is the performance rating for the oil in summer. The change from 20 to 30 would thereby upgrade the oil from being rated for a moderate temperature summer to a little higher temp or hotter longer runs on the highway, the higher the number (20, 30, 40) the higher the temp it is rated for staying viscous and free flowing, so if your car isnt in a pretty hot place in the summer or you aren't sustaining high rpms or highway runs 5w-20 to 5w30 change may not be necessary. Not sure that a supercharger would change anything for oil rating.
Hope that made sense.
I own an '07 GT and run Pennzoil Ultra Full Synthetic 5W-20 and it works fine for me.
Cars are meant to be driven, not trailed.
2007 Vista Blue GT 5spd. JLT CAI. GT500 60mm Throttle Body. Bama Tunes SCT SF3. Bassani X Pipe. Pypes Hi Flow Cats. Magnaflow Axel Backs. MSD Blaster C.O.Ps. Eibach Rear Sway Bar. GT500 Strut Tower Bar. IUP. HID Headlights, Halo HID fogs. Hood Pin Kit. Custom Pinstriping. Red Calipers. Deck Lid Blackout Panel. Chin Splitter. Headlight Splitters. Spinner Center Caps.