Synthetic has greater strength, cleaning ability, lubricity, heat resistance, lower pour point, and on and on and on...
It's also double the price at least, and that's why the factory fill is dino.
Notice how they don't fill it to the full mark, just in-between "add" and "full"? Every drop costs money!!
In all honesty, on a low end machine it's probably a waste of money to use synthetic oil. Most of those tractors fall apart in every other mechanical aspect - steering, deck, spindles, etc - stuff far less lubricated and cleaned than a well maintained engine on conventional oil. So if you are not mechanically inclined, and paying a repairman, the economic life is over far before the engine is done.
However, if you use the machine in very cold weather for snow removal, synthetic does a great job of saving the engine and starter on start up.
I use synthetic in my JD X300. That's just my choice.