The technical advantages of synthetic motor oils include:
Measurably better low- and high-temperature viscosity performance at service temperature extremes
Better chemical & shear stability
Decreased evaporative loss
Resistance to oxidation, thermal breakdown, and oil sludge problems
Extended drain intervals with the environmental benefit of less oil waste.
Improved fuel economy in certain engine configurations.
Better lubrication during extreme cold weather starts
Longer engine life
Superior protection against “ash” and other deposit formation in engine hot spots (in particular in turbochargers and superchargers) for less oil burnoff and reduced chances of damaging oil passageway clogging.
Increased horsepower and torque due to less initial drag on engine.
In general, synthetic oils are derived from more stable molecules and hold up better under stress. This means that they are normally better balanced in their additive package and putting other additives in them can destabilize the mix. The additive package is usually more robust to keep up with the synthetic oil. You’re paying extra for this so why screw with it?
I run 10K to 15K OCI’s with good synthetics and usually add MMO in the final 3000 miles of the run. By that time, the oil is well used and not much can be thrown off in the additive package.