When I acquired the vehicle last summer it has standard fluids in it. The dealership I purchased it from had changed the oil but had put in non synthetic oil. I had done some tune up work on it like changing out plugs, plug wires, distributor cap, rotor, air filter, coolant, and fuel filter but hand't gotten around to changing out any of the fluids before going pheasant hunting. When I left I topped off the tank and filled it up again after going 186 miles and put in 9.3 gallons. On the way back I put 185 miles on it and put in again 9.3 gallons. Shortly there after I manged to get around to changing the gear oil in the transmission and differentials as well as fluid in the transfer case. The engine oil is still the non synthetic fill from the dealership. This time when going deer hunting in colder weather I drove 187 and 183 miles between fill-ups and this time only put in 8.3 gallons each time.
Those 2 gallons of gas alone come close to covering the difference in the cost between standard and synthetic fluids that I put in. With the other driving that has been done the cost of those fluids have now been easily recouped. Granted this is just anecdotal evidence but does help to support the claim of better mileage by using synthetic fluids.
When it is time to change the engine oil I should see more of an increase in mileage but I don't know how much. Now before I change the engine oil to synthetic I will be changing the oil pan gasket and valve cover gasket as they are old and are a bit on the leaky side so that should eliminate any of the associated problems people claim are caused by synthetics.