Change transmission fluid every 30,000 miles. Most owner’s manuals say it isn’t necessary. Yeah, right. That’s why transmission shops are making a fortune replacing burned out automatic transmissions. For optimum protection, change the Transmission Fluid and filter every 30,000 miles.
An automatic transmission creates a lot of internal heat through friction: the friction of the fluid churning inside the torque converter, friction created when the clutch plates engage, and the normal friction created by gears and bearings carrying their loads.
It doesn’t take long for the automatic transmission fluid (ATF) to heat up once the vehicle is in motion. Normal driving will raise fluid temperatures to 175 degrees F., which is the usual temperature range at which most fluids are designed to operate. If fluid temperatures can be held to 175 degrees F., ATF will last almost indefinitely — say up to 100,000 miles.