Background: We recently switched from the original M1 0w40 (MB 229.5 spec) at 100k miles to the new 229.51 spec M1 ESP 5w40 in our '05 e320 cdi. The engine is as happy as ever, and there was seemingly no difference in the way the car ran with the new oil. However...the fuel economy has increased considerably (1-3mpg) and there has been no change other than the oil in the last couple thousand miles. How is this possible? Logic dictates that a similar weight premium synthetic oil should have little to no effect on mileage, especially in a car that doesn't burn a single drop. Even my 2008 Dodge showed little-to-no measurable difference in mileage when switching fron the factory fill 5w20 to M1 0w20 AFE. This is by no means a scientific observation with controls, but you start to notice when the car is getting 20-50 more miles to the tank than it has in the last 100k. I understand that oil that is certified for "Emissions System Protection" essentially will leave less ash if burned up in the engine...but what do the oilmakers change about their oil to ensure it leaves behind less ash? Does it require a different basestock? What ingredients/substances need to be removed from an oil to make it less "ashy"?