Gear oil selection?

[Atlas]

I think I've determined I have AAM axles based on the covers and housing designs. The RPO qr code doesn't list the axle RPO's other than GU5 (3.23). Rockauto seems to indicate the correct oil should be 75w85 AcDelco High Efficiency 10-4091 for SU4/SU7 AAM axles. This is a GL5 oil, whereas the oil listed for "Supplier B" (GM) is part 10-4122 AcDelco Dexron LV, which is a GL4 lubricant.

This is for a 2025 1500 LT Trail Boss.

 

Do I need any LSD additive for the rear? I have G80 but I don't know if that includes "limited slip" or if it's just an inertia locker.

[newdude]

1 hour ago, Atlas said:

I think I've determined I have AAM axles based on the covers and housing designs. The RPO qr code doesn't list the axle RPO's other than GU5 (3.23). Rockauto seems to indicate the correct oil should be 75w85 AcDelco High Efficiency 10-4091 for SU4/SU7 AAM axles. This is a GL5 oil, whereas the oil listed for "Supplier B" (GM) is part 10-4122 AcDelco Dexron LV, which is a GL4 lubricant.

This is for a 2025 1500 LT Trail Boss.

 

Do I need any LSD additive for the rear? I have G80 but I don't know if that includes "limited slip" or if it's just an inertia locker.

 

 

3.0 diesel?  LZ0 3.0 Duramax only uses one rear axle, the 9.75" AAM supplied axle RPO code SU7.  Correct gear oil will be p/n 19300457/10-4091 75w85 high efficiency.  Do not add any additives to the fluid.  

 

Eaton G80 locking diff is a mechanical inertia locker.  

Edited June 18, 2025 by newdude

[Atlas]

3 minutes ago, newdude said:

 

 

3.0 diesel?  LZ0 3.0 Duramax only uses one rear axle, the 9.75" AAM supplied axle RPO code SU7.  Correct gear oil will be p/n 19300457/10-4091 75w85 high efficiency.  Do not add any additives to the fluid.  

 

Eaton G80 locking diff is a mechanical locker.  

 

3.0 diesel/LZ0, correct. Would it also follow that AAM supplied the front axle? Based on pictures I'm pretty sure it's AAM. Same fluid spec?

 

3-ish quarts for the rear, and 2-ish for the front?

Edited June 18, 2025 by Atlas

[newdude]

2 minutes ago, Atlas said:

 

3.0 diesel/LZ0, correct. Would it also follow that AAM supplied the front axle? Based on pictures I'm pretty sure it's AAM. Same fluid spec?

 

 

If the front is also AAM, yes same fluid.  You'd have to run the VIN at the dealer to make sure what front axle it has.  SU4 is AAM, SU5 is GM.  

Edited June 18, 2025 by newdude

[SilveradoRST]

Are you planning to remove the front diff cover to drain or use a pump to drain it?   

[Atlas]

2 hours ago, SilveradoRST said:

Are you planning to remove the front diff cover to drain or use a pump to drain it?   

 

I'd like to pull the covers to have a look, but I haven't gotten the truck up on a lift to really look at things yet.

[SilveradoRST]

1 hour ago, Atlas said:

 

I'd like to pull the covers to have a look, but I haven't gotten the truck up on a lift to really look at things yet.

Come back and let us know.  I’ve watched a few YT videos, some say they can’t get the cover off because the space is too tight.  Others were able to get it off but one guy wasn’t able to get a torque wrench on one bolts, no biggie.  

[redwngr]

GM calls for full synthetic gear lube. 

 

 

[Grover67]

Front cover takes some work to get off and you'll need a gasket as its not reusable, if you have it on a lift it'll help. Just hope most all of the gasket comes off with the cover as its not the easiest to scrape off, not sure what was used when they installed it.

[SilveradoRST]

46 minutes ago, Grover67 said:

Front cover takes some work to get off and you'll need a gasket as its not reusable, if you have it on a lift it'll help. Just hope most all of the gasket comes off with the cover as its not the easiest to scrape off, not sure what was used when they installed it.

How bad would it be if I didn’t take the cover off and just use a pump to drain the front?   I know there’s a magnet on the cover that ideally should get cleaned.  

[PunchT37]

I don`t know if it really matters if it`s a GM or a AAM gearbox. A good full syn 75w90 should be fine. Screw the 85 stuff.

[Atlas]

I ordered some stuff with GM points that landed at the dealer today (wheel liners...can't believe they are 'optional' these days) and decided to ask for the official oil spec on the differentials. I do have the AAM axles according to my VIN, and the spec is 75w85 synthetic GL5, ACDelco part 10-4091 for the 1-quart bottles. The more you know.

 

Next...to get it on a lift and look at access for the front diff cover. I won't need to swap the oil for a while but will order up a gasket or two to keep on hand for when I do.

[Z45]

 

 

Edited June 20, 2025 by Z45

[Grover67]

20 hours ago, SilveradoRST said:

How bad would it be if I didn’t take the cover off and just use a pump to drain the front?   I know there’s a magnet on the cover that ideally should get cleaned.  

 

Its stil going to be pretty tight getting a hose down into the bottom. The vid Z45 posted is pretty much what you're up against. Live I mentioned before about if you do it on a lift it'll be a little easier to work around than doing it off the floor, and yes it would be better to clean it out fully. It will also depend on how many miles you have on it and how much you've used 4WD. Mine was pretty clean when I changed it around 30k

[Z45]

One tip.  Make sure you can remove the fill plugs before draining.  I have seen it happen where the fill plug was extremely difficult to remove especially with aluminum cases,