Yeahhh !! Finally a question about final drive lube.
You can use synthetic GL if you like spending money or you can use plain old GL which will work just fine. The final drive doesn't care.
I think there is data to indicate that crankcase wear will be less over the long haul with synthetic, but that's the crankcase. The tranny might shift better with synth, but that's the tranny. A final drive might last longer with synth over the long haul, but if it's going to fail prematurely, its going to fail and your choice of lube isn't going to make any difference.
If you are concerned about FD failure, the best thing to do as far as lube is concerned is to change it every 6K miles using the same kind of lube (so you have consistency of color) and look for color change in the lube and increasing amounts of metal on the drain magnet. Using regular "dino" GL is cheaper. I regularly use Walmart Tech brand GL.
I've rebuilt quite a few failed final drives that had red colored lube in 'em, dunno who made that red lube, but some of 'em have a jelly-like material in the recesses of the drive that doesn't clean out easily in my parts washer. I don't think that red lube is any better, drives seem to fail just as well with that color.
Anyone who tells you that Brand-X is better has no data to base the claim on. (and, no, "I went 1 million [or however many] miles without a problem" doesn't consititute data. Such reports are "case reports" and have no meaningful statistical value. Kind of like: "I wear a magnetic bracelet and it cured my multiple sclerosis and my diabetes." Or "I killed and elephant with a rock, and here's the rock to prove it."