Autotrans, manual trans and Differentials definitely, show improved wear, heavy truck drive lines have warranties up to 750,000 miles when purchased new and maintained with synthetic oil compared to 250,000 for mineral gear oil.
Engine oil is another story, the base weight is thinner so if there a wet spot with standard oil it'll be leaking with synthetic, personally I use blended oil(synthetic and standard) I do mostly hiway driving and change it at 6,000 miles.