Synthetics along with any other "additives" are nothing more than snake oil as far as MPG goes.
What many vehicle manufacturers have been doing to "help" the mileage is to spec oils which are much wider in cold to heat range. In other words now days 5W-30 or 5W-40 is the "recommended" oil specs. Older vehicles the specs were 10W-30 or 10W-40 or 15W-30, 15W-40.
The idea is the lower the oil spec the thinner the oil is when cold and this "helps" reduce the friction of the cold oil.
Where synthetics "shine" is in longevity, often can go a few miles longer on the oil changes. However the extra premium price of Synthetics sort of takes away this advantage in cost per quart..
If you are looking to "gain" mileage you have several choices... Lighten up on the go pedal OR trade your vehicle in for a "smart car".. But really there is NOTHING you can dump in your oil or gas which will increase mileage inspite of folks who make "claims" that it helps them. SAVE YOUR MONEY and don't bother with marketing gimmicks.
