My mother was getting a regular 19-21MPG (hwy) with an Explorer, using Mobil-1 5W-30. We switched it to Castrol High Mileage (another great oil), for no good reason, really. Since, the MPG has been 17-18mpg (hwy). I understand the Castrol HM is a thick 30wt, and the M1 is a thin 30wt....maybe that possibly has something to do with the matter? Driving conditions/temps were all about equal. Motorcraft and WIX oil filters used with each oil.
After changing to Mobil1 engine oil and the 2 differentals to Mobil 1 gear oil I recorded a 0.3 mpg difference. Both dino oil and synthetic oil were checked over a 2000 miles cycle on my awd Subaru. Yes Syn helps milage but you NEVER recover the cost difference imho.
My Civic was running M1 5w-30 getting 34mpg consistently. I switched to German Castrol syntec 0w-30 and MPG dropped to 30 mpg over 15000 miles on the oil (3 changes). Just recently, decided to go to M1 0w-30 and mileage is back up to 34-35mpg. Some here will say that a 4 mpg drop is impossible just because of an oil. I disagree and my Civic's performance with and without M1 proves it to me at least. Not saying Syntec was not a good oil, just for my small 4 cylinder Honda, it was a little too thick IMO.
My wife's 2000 Civic shows the opposite. She's averaging about 1 MPG more this year running the thicker GC 0w30 than she was last year running the thinner Royal Purple 5w30.
Nothing else has changed that would account for this MPG boost, her driving patterns are the same, and no tune up stuff has been done, she's on the same spark plugs and air filter as last year.
In trying to do my own "controlled" gas mileage tests, I've found that anything less than a 1 mpg difference is really hard to measure CONSISTANTLY. I am leary of people claiming .5 mpg differences, because no one can control wind speed, traffic conditions, weather, etc, all of which can impact mpg.
This issue has been and still is debated on many many automotive forums, but the REAL answer is almost never heard.
The truth of the matter is this: All "true" synthetics have the properties and characteristics (such as a consistent molecular structure, a more frictionless fluid and not thickening in cold temps) to "contribute" to improved fuel mileage, BUT in the real world, there are so many factors that effect fuel mileage, that the ONLY way to positively say that it is the oil (and the synthetic oil alone) that is giving you the mileage difference is in a lab where ALL factors are controlled!
Who has the time or money to prove this out?
Eddie brought up a very good point: if you think synthetics are giving you better fuel economy, it may be such a small improvement that the cost difference is not recovered.
97 Integra (non vtec) with auto tranny getting similar milage to slightly better (less than 1mpg out of 30-32mpg) on Mobil1 5W30.
One thing I do notice is fresher oil gives better milage regardless of kind and grade. I guess there is a diminishing return on running extended drains.