If you have been using the same fluid for the 224k, I would just keep on using the same fluid. At that point if you were to change to something different, then you may start having some problems since the synthetic has a different friction compound.
If you are having problems and want to prolong the life for some time, I would recommend using synthetic trans fluid: Redline, Mobil, Castrol, Amsoil. Not in any special order. They may help some!