I think it's already been stated, possibly by me, but , the original question's answer I believe is at best "maybe" . More accurately, the 5w20 oils have been designed for the engines. In one year the oil specs for a given engine can go from 5w30 to 5w20. The engine type did not change, horsepower is the same, there is probably even some crossover point between the two model years, where the engine in the two came from the same shipment of engines, yet the oil spec is different. CAFE and oil advancement is the reason for the trend, nothing more or less. Just run what you are comfortable with. I'm one of those heavy oil experimenter as well in my '08 stang gt and my '06 4cyl ranger, they love it, and it simplifies my oil purchase, just buy the different filters.