Over time heat in the transmission causes a breakdown of the transmission fluid, wear on the internal transmission components causes a buildup of metal filings in the transmission and condensation can introduce water into the fluid due to exposure of the transmission to external and internal temperature extremes. The recommended fluid change intervals are based on an estimate of when the fluid will have degraded to the point where it is no longer performing well.
Transmissions also contain an internal fluid filter. Make sure the shop doing your transmission fluid replacement also replaces this filter.
vladydady said:
According to the manual a V6 mustang needs the transmission fluid changed every 30,000 miles. Why is that?