Gears & Axles - Would a synthetic get rid of most of my gear noise ?

I read on here once that someone had put a synthetic gear lube in their rear end, and got rid of 95% of their gear noise(from aftermarket gears). I have Richmond 4.10's, and i have gear noise on the highway, but can barley hear it around town. Thing is, I also was told by the guy that installed my gears, that a synthetic will chew up bearings alot faster than a regular oil will, and that he never uses synthetic in a rear. What do you guys think, and what brand and weight of synthetic should I try ?

I've seen guys quite them down by moving up in weight, not necessarily to a synthetic.

My 3.73 richmonds were put in properly and double checked and they whined super loud on the highway.They were totally unacceptable to me..8 to 9/10.We tried running heavier synthetic..richmond recommends their synthetic and went from 80 /90 or something to 80/120..can't remember..no difference or maybe took it from 9 to 8.5..I also dynamatted the entire back of my car double layer..and it maybe took it down to 8/10.I then took off my random tech torque arm and hotchkis lcas and panhard and that didn't help.I finally made my installer /speed shop owner buddy take the richmonds back for credit..they are in his wifes car and noisier than hell in there now..she complains all the time..LOL.I hear redline makes a 240 weight..and though someone said napa had some real heavy weight oil that but can't remember.If that redline don't work then yank those noisy pos richmonds out of your car and make who ever sold them to you give you a refund.
or buy and super loud car stereo like me or run a cutout with no cats in your car maybe.

What is the heaviest weight oil I could run in the rear without trouble ?