* Automatic Transmission: Ford's official recommendation for many automatic transmissions in that era was often a fairly long interval, sometimes even "lifetime" fluid. However, "lifetime" usually translates to the lifespan of the transmission *under ideal conditions*. Harsh driving, towing, frequent stop-and-go traffic, and extreme temperatures significantly shorten the fluid's lifespan. For an automatic, a change every 80,000-100,000 miles is often recommended by many mechanics, but you might consider it as early as 60,000 miles if you drive aggressively or in harsh conditions. A fluid and filter change is usually recommended at the same time.
* Manual Transmission: Manual transmissions generally require less frequent fluid changes. Many experts suggest a change every 30,000-60,000 miles for a manual transmission, depending on driving conditions. However, checking the fluid level and condition regularly is vital.
Crucially: Consult your owner's manual. It will have the most accurate and specific recommendations from Ford for your particular Mustang's transmission type.
Ignoring transmission fluid changes can lead to premature transmission failure, a very expensive repair. Err on the side of caution and have it checked by a mechanic if you're unsure.
