I agree with this. I swapped out my 4T60 in my 94 Cutlass and I stuck with DexII. I bought the Castrol Dex/Merc mix for my recycled 4T60 out of a 95 Z34 and it has been smooth as butter. I did the same on a 95 Grand Cherokee. Chrysler recommends ATF4 but last time I did that, it ate the trans so I use what the trans was built for, so I used the Dex/Merc mix for it too (it called for Mopar fluid or DexII if no Mopar was available). No synthetic additive laced ATF for me. I can't justify the risk.

To the OP, if your shop can use Dex3, then do so. Dex3 is harder to find (either that or he'll probably just use Dex/Merc) but a wise idea would be to add the additive yourself and see what happens, however you may need to drain a bit of the ATF already in the trans to not overfill it.

 

that is the ATF fluid that I would recommend. I can't speak for the additive though, since I've never used it.
Personally I would do the filter and fluid swap first and If there is still an issue with the transmissions performance I would install the Lucas brand transmission additive. I do have experience with that product and I'm satisfied with it's results.

 

The Monte owner I'm trying to help here is the MacGyver type and on a budget. Sure in a perfect world one goes to the $140 transmission fluid / filter change shop like Jiffy Lube etc. and they won't let you bring your own fluids. But seems like at some point, one typically has a shop they have become acquainted with and they put in the parts you bring them. Someone made the comment they would not trust a shop that let you bring your own fluids etc. Hogwash, I have 2 shops who put in what I bring them and both are reputable operations. Stones auto and diesel (been using them since 95, they work on my daughters, girlfriends cars and my truck) and Paul's Auto & 4x4 (since 09, they built my Monte) One just has to find a shop through word of mouth and give them your business and they will be glad to help you save a few dollars, repeat customer.

I did NOT say to use DEX-VI in a bad transmission, but a HIGH MILEAGE transmission fluid. Most of the Jiffy lubes etc. have the standard viscosity fluid for your car. The high mileage fluid is what you need to help you nurse your tranny along for another year or more. So screw what the book says as it does not address changing fluid types for the high mileage trannys, you need to take the initiative and learn a few tricks to get your car or truck further down the road till you have the bank set aside to do a rebuilt tranny.

So for increased line pressure to eliminate the hard shifts and long shifts, slipping use the high mileage tranny fluid and an additional thickener like Lucas Stop-Slip or Lube Guard and keep doing so about twice a year. I use a suction pump to pull out about a gallon of old fluid and add 2 quarts of Lucas and on I go. My 96 GMC has been going on a bad tranny now for at least 3 years now if not longer. Just keep modifying my fluid, don't drive like a madman and down the road I keep going.

The Monte's tranny fluid and filter are extremely easy to change yourself. The funny thing is I can get a rebuilt truck tranny parts / labor to install for cheap $1300, just some weird quirk in me trying to see how long I can get it to last. They are like pretty cheap miles as long as the truck can do it's job. Similar to the episode of Seinfeld where Kramer drives the car past every exit to see how far he can get once the tank is past the empty mark.

 

I got Cottman's 21 point TransCheck and a test drive today. No codes came up and they said everything was within spec. Got the pan drop, fluid/filter change. They said the slight surge was when the transmission goes to full lock past 45mph. So I guess it's fine for now.

 

Cool. Glad you did not have to spend Thousands to get it taken care of!!!!!