I can't speak with any authority, but I did talk to a transmission guy about it. He told me that the main cause of transmission failure is heat (i.e., overheated transmission fluid), and the main cause of heat is a torque converter that is unlocked.
The torque converter (which used to be known as "fluid drive") is the clutch between the engine and the transmission. It is what allows you to sit at a stoplight with the motor rotating and the wheels not rotating. It is a slip mechanism, in other words, and slip causes heat. And heat is wasted energy.
In the bad old days (50's through 80's, more or less), the torque converter always slipped to some extent. However, when gas mileage got be really important, the torque converter was redesigned so that it will slip when you are stopped, but will lock up (no slip) once you are moving. All automatic transmissions operate this way now.
In today's transmissions, the torque converter momentarily unlocks as the transmission shifts from one gear to another. Then it relocks. So if your transmission stays in one gear (such as overdrive) and hardly ever shifts, the torque converter remains locked, and no excess heat is produced. And that's a good thing.
However, if the transmission is constantly shifting back and forth between two gears (such as overdrive and the gear below it), then the torque converter is constantly unlocking and relocking, and each time it unlocks, more heat is generated. This is NOT a good thing. So Blake's Toyota dealer, and Bill's Ford manual, are telling you the right thing. Overdrive is OK as long as it stays in overdrive. But if it starts shifting back and forth and back and forth between overdrive and the gear below, this is bad. In that case, you should turn OD off, and it stays in the gear below.
The tricky part is that if you want to leave overdrive on, you MUST remain aware of the gear changes, so you can tell when the transmission starts to "hunt" - the technical term. If hunting starts, that is the time to defeat the overdrive. But if it shifts only once every 5 or 10 minutes, it is OK to leave the OD on. A tachometer helps in this awareness. Without one, I find that it is too easy to miss the fact that it is hunting.
By the way, the word "overdrive" sounds like some kind of magic word. It is not. Overdrive is just another gear, the top gear. My Ford Explorer has a five-speed transmission, and the 5th gear is called Overdrive. New (2006) Ford Explorers have a 6-speed transmission, and 6th gear is called Overdrive. Many vehicles have a four-speed transmission, and 4th gear is called overdrive. For the purists among us, yes, the term "overdrive" has a specific meaning re gear ratio, but from a layman's perspective, it is just another gear.
Hope this helps
Bill
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