The transmission fluid is a lubricant that helps maintain the transmission of your car working properly: it makes it silent and decreases its wear.
Generally, if your Eos has an automatic transmission, the transmission fluid is changed every 25,000 miles.
If your car has a manual transmission, you should change the transmission fluid approximately every 60,000 miles.
The transmission is next to the engine of your. In the transmission there are smaller parts that rotate at a great speed and transmit a lot of force, which creates heat.
The transmission fluid creates a protection film between the metal pieces, which decreases the friction, cleanses their surfaces and refrigerates them. This protection increases the lifespan of these pieces and makes your car perform better, making it move around more silently and smoothly.
Changing the transmission fluid periodically is also beneficial because of its price: it is always cheaper to invest in the lubrication of the transmission rather than to invest in the replacement of all the pieces because of their wear.