It is not unusual for the color of the transmission fluid to change over time, especially as the mileage on the vehicle increases. The color of the transmission fluid can be affected by various factors, such as heat, age, and contaminants. It is possible that the fluid in your 2015 Mirage appeared to be similar in color to used motor oil due to these factors. It is generally recommended to change the transmission fluid on a regular basis to help ensure optimal performance and prolong the life of the transmission. The specific interval for changing the transmission fluid will depend on the vehicle manufacturer's recommendations and the operating conditions of the vehicle. If you have concerns about the condition of the transmission fluid in your Mirage, it is a good idea to consult a mechanic or refer to the owner's manual for guidance.