My dealer told me that the cost was $300 for the transmission oil change. Compare that to $3,000 for a rebuild. To me the answer is simple, change the oil every 4 years or 100,000 miles (just like you do with the spark plugs). Even if it does nothing, it certainly won't hurt.

As for symptoms, a few are:
- shifting points are off
- sticks in a gear for a prolonged period
- sliping