The reason to use synthetic engine oil is engine and engine component longevity ... the minor improvements in a variety of characteristics like mileage, don't pass the logic test due to the cost delta of the synthetic engine oils. The one exception to this is extremely cold climates - synthetics flow so much better that you can actually START the engines - and, have added benefit, again, of the oil getting to the parts that need it - increasing longevity of parts and the engine.

The cost versus effects curves look different for things like differentials, greasable fits and bearings, and even transmissions, due to the change interval.

George