Should we replace it with synthetic fluid or is it already synthetic? Would synthetic fluid increase differential survivability? What do you think?

Synthetic fluids are more important in high heat environments, like the engine and transmission. I don't really know how hot it gets in the diff. case. Synthetic oils are less prone to 'coking' (burning onto engine parts) again in a high heat environment. I don't think for a differential it would make much of a difference.
Just my opinion.

My 2004 manual states SAE 75W-90 synthetic that meets GM spec #9986115 (GM PN 12378261 in the US).

GM uses Mobil 1 gear synthetic oil for the rear ends on the CTS & CTS-V. Just can not remember the weight.

It would be good to use. Diffs can generate a lot of heat.

My rear end was binding and noisy, the regional rep (whatever his title is) told my dealer to fill / flush / drive 3 times and see if it solves the problem. The binding is gone, but I was told that the original fluid was synthetic and the dealer was told to replace it with conventional (with lsd additive) - it is noted on my repair order. I asked what the life of a rear with dino gear oil is when it was factory filled with synthetic and just got a dumb look. The binding is gone, but the rear is still noisy. I need to go back and ask for either a new ear or an extended warranty for them putting the non-factory fluid in.

Owner of a2003 CTS. I'm interested in changing the rear differential fluid as a preventive maintenace step. Is the producre (and required fluids) the same as the CTS-V procedure posted on the frequently asked question board?

Thanks in advance!

One of the highest failure rate items on the NASCAR short tracks is the rear end gears because of the extreme heat. The next time you watch a race at Martinsville take note of the cars with the rear gear on fire. Synthetic gear oil retains it's lubricating qualities at very high temps and remains fluid at extremely low temps (viscosity stability). Dino oil is fine in warm climates but in extreme cold you won't get the wheels to turn.

Hey people READ your owners manual:

Recommended Fluids and Lubricants

Rear Axle: SAE 75W-90 Synthetic
Axle Lubricant
(GM Part No. U.S. 12378261,
in Canada 10953455) meeting
GM Specification 9986115.