Ford Repair: 97 Merc, Rear Axle oil, synthetic gear oil, friction modifier

Question
About 1000 miles ago I had ring and pinion gear replaced.  The mech. that did the repair said it wasn't necessary to change gear oil after break-in, but other sources say it is.  Now, I'm hearing faint whining between 30 and 60 mph (almost inaudible with passenger window down).  

Do I need to have gear oil changed, and if so, would I need synthetic? Not sure if I have positive traction; some web sites said a synthetic additive was necessary if positive traction.

But this is outside of my expertise

Answer
Jonah-

  It might not be absolutely necessary to change the gear oil after break in, but I'd definitely do it.  The noise you hear is most likely friction, but it could be from a number of things.  If the pinion depth wasn't set properly it could cause the same sound, for instance.  I'd change the gear oil with some synthetic gear oil and add a bottle of Ford's friction modifier.  It's a must for posi-traction differentials, but it's still a good idea without posi.  Hope this helps.

Steve