Rear Diff. fluid

On a 2004 Mercury Grand Marquis did the factory install synthetic oil in the rear diff. when the car was manufactured and when should it be changed under light load use?

The 2004 had a semi-synthetic grease in it from the factory.. The grease should last at least 100k miles. I think Ford recommends changing it at 60k though. It never hurts to change it out. If you have driven through high water, you should change it anyway.