In another recent thread it became a bit more clear that in about 96, the use of any additive to for limited slip differentials was discontinued. My 99 with a G80 (but only 7.625 diff) clearly says NOT to put in any additives.

 

2001ZR2. I tried to search for the thrad by Swartkk but i could not find it.
christine_208. Looks like you have been correct all the time. I just checked the RPO code for Limited Slip and the RPO code is G81 which is not on my car. Just G80 and GT4 for the axels

But it is strange that everybody i have talked to at different parts shops tell me to use SAE 80-90 and add friction modifier when that is wrong. Even a friend of mine that is a car mechanic and he have worked on several race cars for well known drivers here in Europe was sure that i needed to use GL-5 oil with additive as he said.

I am going to use Kendall NS-MP Hypoid gear lubricant, SAE 80W-90, GL-5 oil

I would have been so much more easy if GM could have written in the owners manual for G80 locking diff like they did for standard axle
Axle Lubricant use GM part no 1052271 or eqivalent like SAE 80W-90 GL-5

 

I think my Rear Axle Service thread might be what 2001ZR2 is referencing. The pictures in that thread were from my old 2000 Bravada that had the 7.625 G80 rear end so it will be slightly different in appearance to your 8.5 axle, but the steps should be pretty much the same although I'm not sure if the 8.5 has a fill plug and if it does, if it is located in the same location.

As far as the G80 fluid concerns, for better or worse I've used Valvoline Synthetic 75w90 GL-5 fluid for pretty much all of my 80w90 spec gear lube requirements for the past 20 or so years without issue, including over 8 different vehicles with G80 rear differentials. This includes our diesel trucks that tow a combined weight of 37k lbs on a routine basis in and out of fields using the G80 to keep things moving and have not had a single issue with proper operation of the G80 diff. Granted our diesel trucks have the 14BFF axle so it's not a true comparison to what we're discussing here... I buy 5 gallon buckets of Valvoline 75w90 as it is what gets used in our trailer hubs, mower axles, final drives in our backhoes & small tractors, etc so I go through quite a bit of it. I did splurge and use Mobil 1 in my old Bravada though. ;-)

 

I found what is written below in a truck forum and i think this explains why not to use friction modifier/additive in the G80 Governor Lock very well
The G80 (glove box code) rear end is a governor lock and does not need the friction additive because there are no disks or cones, only the weak locking pin mechanism

A true limited slip needs a friction modifier because they rely on friction disks or cones.


The same goes for how they explained the Limited Slip differentials andhy you have to add friction modifier/additive to the Limited Slip differentials
A true Limited Slip needs a friction modifier because they rely on friction disks or cones.

The G80 (glove box code) rear end is a governor lock and does not need the friction additive because there are no disks or cones, only the weak locking pin mechanism.

I cant understand why GM hwrote the GM part number and type of oil to use n standard axels in the owners manual so why could not GM write the same when it comes to the G80 locking axle?