I don't think there's anything you can do with oil to reduce the noise of those gears, short of packing the final drive full of grease, which isn't recommended.
In theory, I like a little molybdenum in applications like that - transmissions, differentials, etc. When I get around to it, I'm going to switch to 75W90 w/ moly.
Markez, when you say that full synth made your Miata quieter, I presume you mean it reduced the valve noise? I think removing all the oil from a Miata engine would make it quieter, too. After a few days, anyway.