If your transmission already has a small amount of leakage due to old, hardening seals -- then you may want to give this shot before changing over to the synthetic.

Go get a can of your favorite "miracle sealer & rebuild in a can" transmission additive (such as Trans-X). Siphon out an appropriate amount of fluid that will be replaced by the additive - do not overfill your transmission. Pour in the additive and drive your car for 50 - 100 miles. This is just enough time to allow the additive to _just barely_ permeate the seals, yet not long enough to cause them to become too soft (which can cause leakage problems later). Then thoroughly drain off the fluid from everywhere; I know its overkill, but I even drained the Trans cooler & lines. Change the filter & fill with your fresh synthetic fluid and you are good to go.

I did this on a four-speed automatic that had a small amount of seepage from the front pump seal & was starting to slip wildly while flaring from third to fourth gear when it was cold. This was almost two years ago, and that transmission is still working perfectly even with the torque of European spec 5-liter engine applied with a heavy right foot! This little gamble has saved me about $1200 up to now -- I know it will need a proper rebuild in the near future, but as long as it running right...

RTH