I just wanted to get some opinions here. Since new in 2001, I have always run 5w-30 synthetic in my 4.2L V6 F150. Mostly M1 and Royal Purple. The recommended viscosity is 5w-20, however in 2001, 5w-20 was hard to come by. That was my reasoning at the time for the change. So with rising gas prices and my quest for better mileage, I have decided to switch to 5w-20. Saturday morning I changed the oil to Royal Purple 5w-20. So far I notice a bit better mileage, but can't really quanitfy it yet. Anyone think that this may not be a great idea at 80K miles? Should I let this run for 5000 miles and then do a UOA?