hcvone
Full Access Member
I have been using Amsoil since the late 80's, yea I'm old, first oil change my diff's get changed also to Amsoil Severe Gear, been doing this for decades in all my cars and trucks
Rear diff fluid
- Thread starter Tiredmechanic23
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Jolly Roger
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Ill try...
LOTS of heated debate and confusion out there about this. It doesn't help that GM used the G80 RPO code to designate both posi/limited slip differentials (on cars) AND Eaton Gov-Lock locking differentials (found on trucks).
My personal conclusion after spending around 10 hours on the subject reading forum conversations, phone calls, opinions and hypothesis's I could find, and like you said following those links further into there individual rabbit holes......
The G80 Gov-Lock does not require any friction modifier or additive.
GM considers any additive introduced into the G80-GL a contaminate that may cause the diff to fail by delaying the engagement of the locking mechanism (bad) and or causing chattering/shuttering (also bad).
While the Eaton G80 Gov-Lock does have differential clutch plates and works like a limited slip diff, it was not designed to need or use a friction modifier/ additive.
Several service bulletins from GM have been released on this matter.
BUT......
Upon calling a few local shops I found that most if not all of them still use an additive when replacing fluid in a G80 Gov-Loc.
When I asked why they said.... "because its a limited slip differential and that's what limited slip differentials take!"
furthermore, the internet is full of people claiming that they have used a additive in their G80GL and it works just fine.
So I call other lube shops, in other cities and states and received a motley assortment of answers but non that were definitive.
The best answer I got ended up being from an old friend of mine who I had called and left a message with. A regular at the racetrack, the old gear-head type...
He said....
"The reason people are not having problems when an additive is used, is because their not using the locking portion of the differential. Most of these people drive around to work and back and wouldn't know what "diff chatter" was if it bit them in the a$$.
Now people who actually USE their Gov-Locks locking feature, like those who 4x4 or have a heavy foot (especially on loose soil, gravel or snow),..."
He went on to explain that this diff locker works with RPM and the additive (just like the service bulletin said..) will keep the locker from engaging when its supposed to. When and if it DOES engage at a now higher RPM it was designed for... BOOM!
"You now have what has become known as the GM Gov-bomb."
"Run a 75/90 in the rear with no modifier and it'll be good to go!"
He also noted that the reason most "professionals" or lube techs say they run a additive in the G80 is because the "screen" tells them to. Somewhere along the line the confusion of the posi-track G80 RPO code is getting crossed with the Gov-Lock G80 RPO instructions....(imagine that) maybe even at the time the information was written into the system to begin with.
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JuicedLS
TYF Newbie
I use Lucas Oil full synthetic gear lube for both my GL-5 front diff and G80 GT4 rear diff. I replace the fluids every 50k and haven't had any issues yet. 226k+ and still going strong. It works for me so I'll keep using it.
Jolly Roger
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Pretty sure a GL5 is a RPO for "FLEET INCENTIVE CHAMBERS RENT A CAR" not an axle or gear?
FYI to all involved, most if not all of the synthetic 75-90 gear oil from makers like Amsoil, Royal Purple, etc already have FM added. If you look at them closely you will find this. I did a lot of reading and found that RP was highly recommended by plenty of people on several forums and they’ve been running it for years with zero issues. I didn’t even think about it because I wasn’t adding FM. But last week I was looking at the jug and right there in small letters it said “with friction modifier additive”. SMH. I since have read that some believe the GM fluid also has some added, but just don’t add any additional FM separately. I was going to leave the RP in and not worry about it since I have had zero issues with it and I drive HARD and am under boost a lot but I am about to pull my axles you replace the axle bearings do I bought the ACDelco fluid to put in. It’s about the same price and it’s the recommended stuff so WTH.
George B
¯\_(ツ)_/¯
All I can add is that I had a G80 in my 03 Burb that would lock and stay locked for a while or until I put the truck in park. It was annoying in parking lots. A fluid change to Amsoil 75/90 was the cure. I was happy with it.
Here’s a bit of info from Bob Is The Oil Guy about Amsoil. I find it strange that there’s nothing listed on the label like Royal Purple has, nor in Amsoil product info but it seems like it does have it to the tune of 5%. I heard somewhere ( not guaranteed) that the specification GL-4 and GL-5 means there is a certain percentage of LS additive. I also noticed Amsoil’s description of their 75-90 states it’s suitable for limited-slip differentials...
https://www.bobistheoilguy.com/foru...ere-gear-friction-modifier.40314/#post-522215
https://www.bobistheoilguy.com/foru...ere-gear-friction-modifier.40314/#post-522215
JuicedLS
TYF Newbie
Thanks for calling me out on my typo. I was crafting a couple responses for a couple posts at the same time and got all mixed up with what I meant to say. Obviously I don't have a GL-5 front diff or a GL5 RPO code. I meant to say that I used Lucas conventional 80W90 gear oil in the front diff(without the GL-5 friction modifier) and Lucas Full Synthetic 75W90 gear oil in the rear diff(also without the GL-5 friction modifier).
Now I gotta find me a Chambers Rent A Car LS so I can have that GL5 RPO
GL-5 is not friction modifier either. It’s a specification for gear oil determined by the amount of “Extreme Pressure” or EP additive in the oil. GL-5 specification is the recommended gear oil for the G80 differential. More interesting info here and this brings me to the pictures I saw of the gears in @Dantheman1540 ’s differential the other day. Hey Dan, remember those marks on your gears that looked like someone had been welding on them but we all knew nobody would be welding on the damn spider gears? Read this and see if it makes you scratch your head! Oh, and s roll back to the pic I posted of the recommended ACDelco 75-90 I posted above here- it shows the GL-5 spec on the bottle.
08 July 2019
When you are choosing a transmission or gear oil you might be wondering what GL-4 and GL-5 mean and what the differences are between these two classifications. Choosing the right classification is extremely important since they both have different properties and wrongful use could lead to serious damage to your vehicle.
There are four different classifications for transmission/gear oil. These different classifications define a certain level of performance in driven axles and manual gearboxes. They are mainly important for the transport sector in North America, Africa and Asia. The active classifications are:
• GL-1
• GL-4
• GL-5
• MT-1
The higher the Extreme Pressure (EP), the higher the GL category is. It is important to know that the SAE Motor Oil Viscosity Chart (J300) is different from the SAE Gear Oil Chart (J306). The viscosity has no relation with the API Gear Oil Classification and should be chosen based on the recommendations from the manufacturer based on the temperatures where you will drive.
The main difference between GL-4 and GL-5 gear oils is the amount of EP additives. Sulphur/Phosphorus containing products are used as EP-additive. This additive has the purpose to prevent the occurrence of micro-welds on the gear flanks at the local high temperatures which prevail in EP circumstances (temperatures well in excess of 800℃!) GL-5 has roughly twice the amount of EP additives compared to GL-4, which is why it is often used in high-pressure circumstances such as in a front axle and rear axle differential.
Sulphur/Phosphorus additives however have an unfavourable property: they can react aggressively towards bronze and copper. This can be disastrous for the synchromesh rings of a gearbox. Therefore it is not recommended to use GL-5 in a gearbox unless the manufacturer allows this.
To conclude:
• GL-4 is suitable for hypoid gear service when they are under severe service but are without shock loading.
• GL-5 is suitable for hypoid gear service under severe service and shock loads and not for use in a gearbox.
Dantheman1540
Full Access Member
GL-5 is not friction modifier either. It’s a specification for gear oil determined by the amount of “Extreme Pressure” or EP additive in the oil. GL-5 specification is the recommended gear oil for the G80 differential. More interesting info here and this brings me to the pictures I saw of the gears in @Dantheman1540 ’s differential the other day. Hey Dan, remember those marks on your gears that looked like someone had been welding on them but we all knew nobody would be welding on the damn spider gears? Read this and see if it makes you scratch your head! Oh, and s roll back to the pic I posted of the recommended ACDelco 75-90 I posted above here- it shows the GL-5 spec on the bottle.
08 July 2019
When you are choosing a transmission or gear oil you might be wondering what GL-4 and GL-5 mean and what the differences are between these two classifications. Choosing the right classification is extremely important since they both have different properties and wrongful use could lead to serious damage to your vehicle.
There are four different classifications for transmission/gear oil. These different classifications define a certain level of performance in driven axles and manual gearboxes. They are mainly important for the transport sector in North America, Africa and Asia. The active classifications are:
• GL-1
• GL-4
• GL-5
• MT-1
The higher the Extreme Pressure (EP), the higher the GL category is. It is important to know that the SAE Motor Oil Viscosity Chart (J300) is different from the SAE Gear Oil Chart (J306). The viscosity has no relation with the API Gear Oil Classification and should be chosen based on the recommendations from the manufacturer based on the temperatures where you will drive.
The main difference between GL-4 and GL-5 gear oils is the amount of EP additives. Sulphur/Phosphorus containing products are used as EP-additive. This additive has the purpose to prevent the occurrence of micro-welds on the gear flanks at the local high temperatures which prevail in EP circumstances (temperatures well in excess of 800℃!) GL-5 has roughly twice the amount of EP additives compared to GL-4, which is why it is often used in high-pressure circumstances such as in a front axle and rear axle differential.
Sulphur/Phosphorus additives however have an unfavourable property: they can react aggressively towards bronze and copper. This can be disastrous for the synchromesh rings of a gearbox. Therefore it is not recommended to use GL-5 in a gearbox unless the manufacturer allows this.
To conclude:
• GL-4 is suitable for hypoid gear service when they are under severe service but are without shock loading.
• GL-5 is suitable for hypoid gear service under severe service and shock loads and not for use in a gearbox.
Significant time was put into this nicely done as always!
That is a very good thought about those tore up spider gears. When I pulled the Vin on the truck it appeared that it was equipped with a manual trans from the factory which would explain the damage being caused by the shock load of a clutch dump or harsh shift.
I have never looked this deep at GL4 or 5 fluid and tomorrow I'll have to go check the bottle of what's in my Tahoe 9.5sf
Somewhat off topic but NV4500s are very specific about what fluid they use and a typical GL4-5 is not allowed whatsoever. Caught me off guard because most other manual trans I've had use a GL5 oil.
