Gear lube question

Campfire Time wrote:
The G-80 is not Limited slip, it's a locking differential. Still it needs the modifier.

Check your manual for the correct part # for the gear oil. The specified GM gear oil should already have the modifier mixed in. If you use Mobile One look for LS on the bottle. You would have to use the modifier if and only if you bought fluid without it.

I've had 2 trucks with the G80 and used Mobil One 75W-90 LS. Never an issue.

true, it is not a limited slip, but it does have clutch plates to help enable the locking function. That's why the factory lube has some friction modifiers, but to much/to little can cause problems.

There are TSB's talking about things like chatter/etc. related to using lubes with the wrong combo of modifiers.

that's why I've just stayed with GM "grape juice", readily avaiable from amazon at a better price than the dealer. IIRC the G80 will take about 4 quarts.

RCMAN46 wrote:

GeoBoy wrote:
On the transfer case you should overfill it by 1 quart. The way to do that is by removing the switch on the top of the case towards the rear of the case and adding the additional oil there.

What is the purpose of overfilling by 1 qt?

My understanding is that the transfer case used in his truck would run hot and literally cook the lube to a vapor. They actually said to change to 5W-30 oil but I ran the Torque-Drive synthetic fluid recommended for Allison transmissions and added the extra quart. Never had a leak from the overfill. My transfer case was a manual shift case.

GeoBoy wrote:
On the transfer case you should overfill it by 1 quart. The way to do that is by removing the switch on the top of the case towards the rear of the case and adding the additional oil there.

What is the purpose of overfilling by 1 qt?

CampingN.C. wrote:
"Originally Posted by General Motors

LOCKING DIFFERENTIAL LUBRICANT (SERVICE INFORMATION) #91-4-109
SUBJECT: LOCKING DIFFERENTIAL (G80) LUBRICANT - (SERVICE INFORMATION)
VEHICLES AFFECTED: ALL LIGHT TRUCKS EQUIPPED WITH G80 REAR AXLE ALL YEARS
Some light duty trucks equipped with locking rear axles (G80) may exhibit rear axle chatter, especially when turning a corner from a stop.
This condition of alternate engagement and disengagement of clutches in differential assembly is usually caused by contaminated axle lubricant.
To correct this condition, drain and refill the rear axle with SAE 80W-90 GL5 (P/N 10950849).

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.

Exactly. This says you should not use an additive to thier specified gear oil. If you read up on GM's part number for this (the gear oil that is), as well as the specs on the G80, they do actually use an additive so you don't have to. Takes out the guess work.

"Originally Posted by General Motors

LOCKING DIFFERENTIAL LUBRICANT (SERVICE INFORMATION) #91-4-109
SUBJECT: LOCKING DIFFERENTIAL (G80) LUBRICANT - (SERVICE INFORMATION)
VEHICLES AFFECTED: ALL LIGHT TRUCKS EQUIPPED WITH G80 REAR AXLE ALL YEARS
Some light duty trucks equipped with locking rear axles (G80) may exhibit rear axle chatter, especially when turning a corner from a stop.
This condition of alternate engagement and disengagement of clutches in differential assembly is usually caused by contaminated axle lubricant.
To correct this condition, drain and refill the rear axle with SAE 80W-90 GL5 (P/N 10950849).

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.

The G-80 is not Limited slip, it's a locking differential. Still it needs the modifier.

Check your manual for the correct part # for the gear oil. The specified GM gear oil should already have the modifier mixed in. If you use Mobile One look for LS on the bottle. You would have to use the modifier if and only if you bought fluid without it.

I've had 2 trucks with the G80 and used Mobil One 75W-90 LS. Never an issue.

GeoBoy wrote:
I use Amsoil Severe Gear 75W-90 in my GM's and added their Slip Lock without any problems. If you read the spec's on Severe Gear, I don't believe you need any additives for the G80 differential. Don't forget the front differential if it is 4x4 and don't forget the transfer case.

I agree, AMZ/OIL Severe Gear is very highly rated. Here is a pic with 65K and towing 28-29K combined with my 11 HO RAM Dually, Fluid had at least 50K on it. The teeth om the ring gear had not even worn smooth yet. I run the 75/110 as recommended by AMZ/OIL.

the G80 is pickier about lube than a std open diff. There are issue that can arise depending on what friction modifiers are in (or not in) the lube. the G80 has a set of clutches to help enable the true auto lock function.

Best bet is to use the GM "grape juice" as GM specifies.

but folks have had good luck with some other lubes as well.

I just stick with the GM grape juice, available on amazon at a good price.

I've used M1 or RP in all 3 of my G80 equipped trucks going back to '98

No issues with any of them.

On the transfer case you should overfill it by 1 quart. The way to do that is by removing the switch on the top of the case towards the rear of the case and adding the additional oil there.