Many manufacturers recommend a transmission flush every 30,000 miles or 2 years. However, not all recommend this frequently some suggest a flush only every 100,000 miles, and others don't recommend a transmission flush at all. A transmission flush can extend the life of your transmission.
Is it bad to change transmission fluid on high mileage?
Changing the transmission fluid on a high-mileage car is risky. Don't do it unless your transmission is running fine and the fluid is first-rate, because it can make transmission failure imminent.
Why You Should Never flush your transmission?
Transmission fluid is highly detergent which can wash the varnish off clutches, causing it to slip. Pressure flushing can cause aging seals to start leaking. When it leaks more than a quart it could burn up the unit.
Is it better to flush or change transmission fluid?
A transmission fluid change will help to restore your system to good working order and is the cheaper option. It is also a relatively simple task that can be undertaken by vehicle owners. A transmission fluid flush is more costly, but will replace all of the fluid and any contaminants that have built up in the system.