No, I switched from the regular Amsoil 10w30 to the S2K 0w30 and my mileage went down, from 27.7 to 23.9 highway. City mileage maybe went slightly higher. I don't think the decrease had anything to do with the oil, BTW. I do like the 0w30 for how the engine "felt."
IMO, I don't believe motor oil can significantly affect mileage. Other factors are more important, like tire pressure, weather, or air conditioning use. Isn't an "energy saving oil" defined as an incremental mpg savings, say 0.2 mpg? I'd like to see a dynometer (sp?) show a 1 or more MPG increase before I'd believe it.