I need to change the transmission fluid on a '99 mercury villager. More than one person has recommended against "flushing" or "backflushing" it.

They tell me that the best thing to do is just drop the pan, let the fluid drain out completely, change the filter, put the pan back on then refill the fluid.

What is the downside to flushing it?

Currently, we are getting a several second "pause" before the car goes into Drive after moving the lever from Park to Drive.

... so I'm hoping that changing the fluid and filter will help.

Further, is there any sort of additive that you'd recommend to add to the transmission fluid that might help?