After much research on various forums, including the Holden forums, it has been determined that the "best" is actually the "only" lube that can be used on our G8's with the stock non-camaro rear. Apparently only the G8 and the Land Rover "E" or Electronic Differential uses this special Carbon Modified gear lube. It is 75w-85 fully synthetic gear lube with special modifiers/additives for our carbon coated parts. The fluid is made by Castrol. It is called: Castrol SAF Carbon Mod Plus, it does not cross to any other type fluid or brand. The diff has carbon clutch discs. The castrol lube is is designed for our type of diff specifically. Land Rover part number is LRN #500070 and is roughly 60 bucks a qt. GM part number is # 92184900. Locally it was 40 bucks a qt from my Pontiac/GM/Buick dealer. It came in a new bottle exp dated to 2/14 and mentions to be used for carbon coated rears and is back compatible (though expensive). I asked for a discount and he gave me 15% off just for the asking! Unfortunately it does require two bottles because it uses just 3-6 oz of the 2nd bottle.
I have78,000 hard miles on my car.
When mine came out it was BLACK AS PITCH, and oddly much thicker then what went in, which BTW was nearly clear like apple juice, and much thinner then what came out. I plan on getting another bottle and replacing this fluid again in like 3 months, I will essentially use this change as a flush since the out going lube was so dark black. SOTP feeling was that it was very quiet, did not realize my rear was making any noise until it was gone, a real smooth feeling, like driving on silk. And it seems I really did get a bit more mpg, though complete testing is continuing. I figure 1/3 to .5 of a mpg. I do reccomend a change of the rear lube around 50,000 miles or so. My mechanic took roughly an hour to do in the driveway.