tranny fluid like all the fluids in the car have additives. over time the additives break down and reduce the perfomance of the fluid. tranny fluid has additives to help disperse heat for one, heat is a major cause of trannys. heat breaks down the aditives faster which causes more heat etc. if the tranny isnt slipping, there are some things you can do, there is a service called a T-TECH which changes almost 100% of the tranny fluid by intercepting the tranny line and using the tranny pump to push the old fluid into a machine and the new fluid runs back to the tranny. but the filter in the tranny should be changed too, and if there is a magnet in the pan it may be full too. i work in a shop that does these. i have seen some cars come in with slipping, and after the t-tech it stopped, ( we are told to check for slipping before we do the service, then they have to sign a waiver cause it may not help) as far as synthethic, if you have any leaks at all, it could make them worse. synthetic additives dont break down as easy but it also may "clean" spots of build up in high milage components causing a leak. more in engines than trannys though.
if it was me, i would drop the pan, change the filter, clean the magnet and drain then get the 100% fluid change service. (with non syn ATF) but if you do, make sure you recheck the level after you've driven it for a few hours. the new fluid going in is cold and ATF expands when hot.
