Transmission

Sorry to say the tranission might need a rebuild soon but that said..
Id say just change it with all new fluide and a .new filter and have the cooler completely flushed too.
Also Just onky adding some might make it worse becuase you're just leaving the dirty oil in there wich is what's making it slip to begin with
So Completly flushing it is the best thing you could do right now if its not to late allready.
The good thing is
In this case where it just recently started slightly slipping then theres no better time to completely flush it than now..
it will improve we just can't say how much but it will improve
If you do get it all flushed really clean than the seals inside will seal beter and might help stop some if not all of the slipping but only right now..and only if the slip is minor and is just starting to happen ..and
becuase your saying just started its so early in the wearing process..
Its better to do it than not do it
And if it doesnt completly fix it youl know it was to far gone to save ..either way it will improve to some degree
And at least you did the right thing ..thats the good "
Now heres the bad "
If you dont though it will
Most certainly fail without a doubt . ..so yeah id say go have it completely flushed to remove all the contaminents not just drained and refilled but completely flushed and have all new fluide and the filter replaced asap. ..good luck hope this helps

The hardest part of checking the fluids in most of the "sealed" transmissions is usually getting the vehicle high enough that you can get under it while keeping it level. You cannot just jack up one end or side. So if you have a way to do that safely then from there it is easy. First you locate the fill check plug on the side of the transmission. It uses a square drive tool # DW260-070 or DT-46475. Then you follow a simple process for most of them. Drive around or idle the engine to bring the transmission fluid up to about 90 degrees, then elevate the car and put a drain pan under it. Get a quart or two of the correct fluid for the transmission (Total ATF H50235 or Esso LT 71141 OR an equivalent synthetic ATF ) Next you run the engine and hold the brakes while you shift it through each gear, pausing in each one about 10 seconds. This ensures that the fluids are in the various passages and lines. Now shut the engine off and go underneath. Loosen the drain plug, wipe the crud around it off, and remove it, SLOWLY. If fluid starts to leak out, it is full, let some leak out and look at it for color and smell, if it is dark brown or black and smells like burnt toast it should be replaced. If it doesn't have any come out you have a choice, add some new until it does leak out the fill or to drain all of it and add new fluid, be advised that you cannot get all of the old fluid out without multiple fills because of the fluid that stays in the converter and lines.
If you decide to do a full service then you find the drain plug at the bottom of the case, it uses the same tool as the fill plug. You do the same steps of driving it around and lifting it prior to draining it. Remove the fill plug first, then the drain and let it run out. Once it stops dripping out, reinstall the plug to 33 ft lb. Then fill until it runs out the fill. Put the fill plug in and start the engine and run it through the gears, now check the level again. Top up as needed. Now reinstall the fill plug to 33 ft lb. See if the slipping has changed. If the fluid that came out was black and smelled burnt a fluid change probably won't do much as those are typically caused by the clutch facings getting charred and worn. That transmission does have a filter, but replacement requires the transmission be removed and the case split to access it, if you are at that point, it's not a lot farther to a full rebuild and if it is still slipping after the fluid change I would look at either a rebuild or a remanufactured unit, IF the rest of the vehicle is in good enough condition to warrant that.