Stop. STOP! The misinformation here is getting out of control ...
First and foremost, what kind of diff does the vehicle have? I am the first to admit I don't memorize every vehicle option from every OEM, so I have no ability to tell you what is in the truck.
However ...
I can speak to part of this issue that is so often misunderstood. The G-80 is the RPO option code for the "locking" differential from GM. It applies to the axles, regarless of size or application. It is a code to designate the use of a traction control mechanism; it is NOT specific to one differential part number. Therefore, you'll see the G-80 listed in 1/2 ton, 3/4 ton, and 1 ton trucks. They may not use the same diff, but they will be using a similar traction device.
The "G-80" most certainly does use clutches in it, and it is not a "locker" in the sense that it has direct mechanical bond between axles, such as an "air locker" or "e locker". The G-80 diff uses a speed sensitive flyweight system and pawl to engage or disengage a clutch pack very similar to a "limited slip" differential. You can google Eatons site and find an old video presentation on it with some efforts. But as sure as the sun rises, there are clutches in there! And it acts VERY similar to a traditional LSD. In fact, the G-80 is really nothing but a glorified LSD, in that it is only active part-time when conditions are met (above a certain wheel-to-wheel rpm delta but below a predetermined vehicle speed).
Here is a direct quote from an engineer at Eaton; it is several years old now but still just as valid:
Automatic Locking Differential Lubricants. [Note from Michael Asmussen, Torque Control Products Division of Eaton Corp.] We recommend the following lubrications for our locking differentials:
1)Texaco 2276; Synthetic 75 W90; Gm Part # 9986115
2)Texaco 9622; Mineral based 80W90; GM Part # 9985290
3)Texaco 2080; Synthetic 75W140 (heavy duty applications); GM part # 9985991
Note - All of the above lubes are preblended with friction modifier. No additional modifiers are necessary or recommended. As far as other lubes are concerned, any standard GL 5 lube will work, but the units perform optimally with the three listed above.
You will note that he specifically mentions the inclusion of "friction modifier" in the Texaco fluids, which GM/Eaton was using at the time. He clearly states that any GL-5 fluid will work well, so you don't "need" a synthetic fluid; dino's will work just fine. He even lists the mineral based fluid as one of the approved GM part numbers. But the reason he specifically mentions FMs is because "Looky, looky... there be clutches in there, Ma!"
As for the topic of FM, Amsoil actually has a nice write up here:
http://www.amsoil.com/techservicesbullet...%20revision.pdf
As you can read for yourself, the use of GM spec 9986115 is simply the OTC version of a 75w-90 syn with FM already added. That is why they state to not add FM; it's already in there! Too much FM is just as bad (if not worse) than not enough FM. Either way, the clutch pack won't operate correctly.
Whether or not you use syns is up to you.
They are not needed nor required by GM. Syns are "recommended" but not "required" - make note of that! If you replace the fluid, and you have a G-80 diff, you will need some portion of FM for the proper operation of the "locker" (which really isn't a locker, but a fancy part-time LSD). I will also note that there are many dino fluids that pass the SAE J2360 test, which is a civilian version of the old MIL standard. These J-2360 fluids are even "better" than a sole GL-5 designation, because they speak to performance, as well as blend characteristics. IOW - many J2360 fluids will also meet GL-5, but not all GL-5s will meet J2360.
But the Eaton diffs are pretty robust, and just about any GL-5 will suffice well more than enough.
'Nuff said.