I am using Mobil 1 0W30 in our two new vehicles (Toyota Sienna and Corolla) to keep the warranty intact, and changing when the indicator light comes on (5K miles) Neither vehicle consumes any, and it stays clean.
Both started well this winter with a protracted cold spell that hit this winter, a lot of mornings below -20F, one at -34F and one at -40F. (Actually drove the old Escort on the -40 morning, it uses 0W20 Mobil 1
I have no reason to question the claimed 2% or so increased mileage, but doubt anyone gets consistent enough mileage to ever prove it outside the laboratory. The Sienna gets 27-28 mpg on the highway when going with the wind, 22-23 going in to it, and about 18 mpg when towing.- that pretty well buries a 2% change!
I will probably switch both vehicles to Mobil 1 0W20 and 20K changes after the warranty runs out, as that strategy has consistently taken our cars to the 300K mark and burn no oil and run like new - And my vehicles are used for "extreme" towing!!