I recently serviced my transmission (at 230K) because I was having some minor surging in 4th gear that I thought was transmission related. My trans fluid was pretty dark as well even though I had just had it serviced at 200K or 18 months ago. FYI, during the servicing, I found little or no metal in the magnets in the reservoir. After the service, I began having some hard shifting and slipping issues when my transmission is cold. After some reading and watching some videos online (Chris Fix has some good ones), I decided to add some Lucas Transmission Slip Fix. Since its cold right now in Texas, it was a bit difficult to get the gunk down into the transmission even with it warm. I added about 8oz (guesstimate) and it improved the overall shift and slippage so I added another 8oz and then added about half a quart of trans fluid (I was about a quart low to start with). After the second dose, I took off on a cold morning and had serious shifting problems. So much so, I took it home and drove another car to work. After work, I took it out for another spin and as it warmed up it got much better. I ran it manually through all the gears multiple times and each time it got smoother. Finally, I did some tests. If I manually shifted at around 3000rpm, every gear works great, no slippage. If I let it shift on its own, each shift is delayed (opinion) and it thunks hard into each higher gear. So my question for the forum is this. Does this sound like an issue with the trans or with the torque converter?

 

I'd say trans for sure. Slipping on takeoff wouldn't have anything to do with the converter as it would be unlocked. Probably also what burned up the fluid in 30k miles.

That Lucas additive is just intended to be a temporary fix to prolong the inevitable.

IMO if that's the original untouched trans, it's way overdue for a rebuild anyways. You'll just have to make the call if you want to spend the money on a rebuild or scrap the car.