Synthetic transmission oils are especially recommended for applications with high thermal stress (e.g. demanding topography, sporty driving style, transmission with retarder). Thanks to their flatter viscosity-temperature curve they are also ideally suited for use in regions with a cold or hot climate. Also, the additives used are often of a better quality, which, for instance, translates into better protection of bearings and gears.
The argument that the more frequent oil changes required with cheaper mineral oils help remove dirt from the transmission, thus increasing its service life, is only valid in exceptional cases (e.g. use in the desert). ZF transmission oils are synthetic oils, which undergo an elaborate filtration process during the factory fill. Thus, under no circumstances should ZF synthetic factory fill oils be changed prematurely.
Oil of an inferior quality can be inadequate from the onset, leading to wear and dirt accumulation. Especially for high oil temperatures, the general advice is to use synthetic products.