There is just too many "modular" motors that have been serviced with 5W-20 that have XXX,000 miles that have used nothing but 5W-20 in the engines lifetime for me to buy the "theory" that 5W-20 is inferior to other grades. But I will advise if you are experiencing consumption, constant wide open throttle(WOT) driving conditions, your engine is modified for higher performance, or that you are losing sleep over using a 5W-20 oil you can go to a higher viscosity. Some modular engines that are used as police cruisers, taxi's, and other "fleet" service have everything from 5W-20 to 15W-40 viscosity and are fine. The 4.6 and 5.4 family has proven to be a very durable engine. So in short you will have to determine for your self what viscosity to use. For my driving I will just continue to use conventional 5W-20 until my driving habits change or if my engine starts to consume oil as it ages.