I know Lucas has an additive that pretty much is for worn seals. It works. I had a 604 that had 2nd and reverse. PUt the stuff in it drove up and down the road for about 15 minutes, parked the car overnight, and the next day had all 4 gears. That was about 4,000 miles ago. transmission still has a axle snappin 1-2 shift once in awhile, but overall it did the trick, for that. I think they have an additive for slipping trans, but not for sure.