[BNaylor]

Running up to 8 oz of Seafoam tranny additive for a very short period of time won't hurt. As you can see from the link it does a good job of cleaning out the transaxle internals.

If your GTP is used as a daily driver best to stay with the Dex VI or Dex III. I use the Royal Purple Max ATF because my GTP is not a daily driver and used primarily for drag racing, ET Bracket. It is a high performance full synthetic ATF that meets the Dex III standard and runs about $10 per quart. Superior heat and friction so it is excellent for racing applications or heavy duty use. The Dex VI is a synthetic blend.

Also, I would highly recommend adding the accumulator shift kit mentioned.