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Discussion Starter · #1 ·
Now I do know how to change the oil, I'm just curious as to what oil to buy. I drive a 2005 Silverado 4x4, 4:10 gear ratio. I was told I might need synthetic 75w 90, a new gasket (I dont have a drain plug so I need to take the whole cover off) and possibly rear posi additive. Someone with knowledge to this would be a huge help. Plan on doing it tonight with a few wobbly pops
 

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2021 GMC Sierra Denali CC 4WD Ultimate 6.2L
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75W-90 Synthetic Gear Lube is required, OEM fill is 75W-90 Synthetic. Yes, you will need a new cover gasket as well. Should need between 2-3 qts to do the rear.

NO ADDITIVES ARE TO BE USED, trucks DO NOT have a posi rear. They have locking rear axle and do not require the use of any additives.
 

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Discussion Starter · #3 ·
Thanks for the swift reply. I'm just curious about the posi additive because my brother swears I do need it. He has a 2000 Silverado that he claims when he didn't put any additive in, it blew up his rear diff and it was a super costly repair. What would happen if I were to add it?
 

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The use of any additive in locking rear axles (G80) is not recommended. Rear axle additives are designed for use in limited slip differentials which are normally installed in cars. All light duty trucks equipped with RPO G80 make use of a locking differential and the use of additives will delay the engagement of the locking mechanism and may decrease axle life.
 

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Discussion Starter · #5 ·
In my glovebox it has a code for G80, or it should, is what you're saying?

What is a posi anyhow, what is it good for and how is it used? What does a locking rear differential do?
 

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Posi which is short for Positraction, a Chevrolet Name from the 60's-70's muscle car era and refers to a Limited Slip Differential as explained in the video.

Cars use Limited Slip
Truck and SUV use Lockers exc for the discontinued Quadrasteer (QS) models, they use a Dana axle and the posi code was G86.

Both the limited slip and the locker carry the same option code, G80. When people look up G80 in the online rpo list they always find posi or limited slip. Option codes have different meanings on difference vehicles.

If you do not have G80 or in the case of the QS G86 code on the SPID label then you have an open differential.
 

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Discussion Starter · #9 ·
Wow have you ever been helpful. Thank you very much!

While we're on the topic; my 4x4 has been quite wonky lately. The push buttons do not light up every time, so I tried a new one and still didn't work. It occasionally shifts itself into 4 low overnight so I simply change it back. The buttons don't work at all sometimes and no lights are on, cannot shift into any other drive. My service 4x4 message comes on every now and again. So I'm not sure if all it needs is an oilchange? Or maybe it's the 4x4 actuator?
 

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I have helped 100's of customer get the right fluids for their vehicles and the G80 confusion comes up often mostly due to incorrect info spread in web forums. That and owning about 1/2 doz GM trucks in the last 20 yrs.

Do you have the 4 button switch with the AUTO button on the top, or the 3 botton switch? The AUTO is the autotrac transfer case and a little more costly when it comes to parts and requires special fluid. The 3 button is just a regular TC that is electrically shifted and uses Dexron III ATF (not dexron VI). The electric switches and motors are prone to failure due to shoddy electronics supplied to GM.

Its not the lube that would cause those symptoms, there are several parts to the 4x4 system, the dash switch and encoder motor in the TC. The motor that engage the transfer case can get real costly so you need to determine the problem before spending money on parts that are not bad.
 

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Discussion Starter · #11 ·
I'm very thankful, it's much appreciated. I have the 4 button and I just changed the TC fluid about a month ago
 

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Discussion Starter · #12 ·
I figure it's the actuator or motor in the TC because I tried a new dash switch with the exact same results. What do you figure?
 

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The rear diff gasket is made of rubber on steel and is designed to be reused.
Also, many brands of gear lube will already have the posi limited slip additive in them and can be safely used in both open differentials and the G80.
This way, lube manufacturers don't have to worry about having three different gear oils; one for open diffs, one for limited slips, and one for lockers.
The actuator mechanism in a G80 uses a spring loaded pawl and works off centrifugal force. If you use a brand like Mobil 1 that has the additive in it already it will have no effect on lockup.
 

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wuurbz said:
I figure it's the actuator or motor in the TC because I tried a new dash switch with the exact same results. What do you figure?
I hoped you used the AUTOTRAC II fluid when you changed the AUTO transfer case. Any other fluid might cause irratic behavior.

Those 2 items can cost in the 100's of $ so you better be sure which one is bad before replacing them. I have never had to replace those items so I am at a loss to offer any advice. It migh be wise to have the dealer determine the problem, could save you money in the long run.

I see you are in Edmonton, Alberta CAN. If you want to use AMSOIL Gear Lube, PM me and I will tell you where to go buy it in Edmonton right now.....
 
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