My 2004 Elantra has now reached 35K. It tough it would be a good idea to service my A/T.
Some forums on the web says to flush the whole thing other just to empty the pan and level up with new fluid.
I decided to go with my dealer recommendation (this is what he does when you ask for an A/T service):
drain the pan (approx. 4qts) and flush the transmission cooler. So here it goes:

1- Drive your car to heat up the A/T Fluid
2- Remove the drain plug. You'll need a 24mm box to remove the plug.


3- If you want to you can lift your right rear to empty as much as you can
4- Thoroughly clean the plug to remove all metal particles stuck to the plug magnet and be sure to replace the seal.
As you cans see on the following picture the seal gets corroded with time.


5- Put the plug back in place and do not over-tight.
The pan is made out of an aluminum casting and the treads can easily be damaged.

6- Next step is to flush the cooling circuit. To do this I bought the stuff Hyundai recommends.

Start by removing both cooling hoses. Place a container under one line and connect the cooler flush to the other line.
Flush the line until the liquid coming out is clear.
Swap the lines and flush again.
Turn your cooler flush bottle upside-down and flush until air comes out.
Put the hoses back in place. You're done with this.


7- Fill it up! You will need SP-III fluid to do so. This is the only fluid recommended by Hyundai.
Other fluid MAY be suited for this A/T but if you want my advice the saving ($) you may do (or not)
is not worth taking the risk to ruin your transmission.
Be sure your car is leveled up, start your engine and place your transmission in neutral. Fill it up until you hit the top COLD marker.
Do a test drive to heat everything up and re-check with the WARM marker. Adjust level to be within the WARM marker zone.


You're now good for an other 35K. This service is what people would refer as a "Makeup" fluid change since we're only doing a partial change.
The transmission can hold up to 10qts but we're only changing 4qts. Replacing 4qts of fluid brings new additives in you A/T making sure
it runs as it should and protecting it from deterioration.
Doing so every 35K should keep your A/T in shape for the life of your vehicle.