I've been experimenting for years with oil and i have to say that i've never been able to see any difference in fuel economy between oil grades and types. In fact my best tanks were on 15W40 and my worst ones on 5W30. Currently experimenting with higher tire pressure and this on the other hand can make a tremendous difference. I don't recommend it but i'm currently at 48 psi rear and 39 psi front (above Mercedes spec) and i've gone from 8.5L/100 km to about 7.5L/100 km so far. I use various hypermilling techniques and never drive above 60 mph. All highway, no short trips.