I recently changed the rear differential oil (Ford Ranger 1997 2WD V6 3.0L). I put some 80W90 synthetic gear oil in there. In my mind, when using synthetic...you're suppose to be clear for a while.

Wrong. few weeks later, a noise starts from the rear during tight turn. I saw some articles about "friction modifier" to add to diff oil.

How does it work. Why do I need to put that in a "already additive well balance" synthetic gear oil? Will this friction modifier solve this problem ? During this time, do I allow excess wear to rear diff components or this is just noise?

Why do not design mechanical devices in compliance with actual lubricant available on the market, especially synthetic ones?

A lot of questions...now, what you think about my problem!

do you have ls rear end ? if so then you need to add the friction modifier for the clutch pack thats why when turning you hear the rear end if you have a one legger non limited slip rear end then you might have a pinion and ring wearing out or a axle bearing gone bad does the noise come and go when applying gas and when back off on coast ?