Fluid Change Questions

[Core-Lokt]

I'm going to change the fluids in both differentials and the transfer case. If anybody knows specs/capacities on these, could you post them? And is there a friction additive for the rear differential. Thanks in advance.

[Another JR]

I changed the fluid in the rear axle of my 2021 gas 3500 hd last Fall at 15k miles. I used Amsoil Severe Gear 75w-90 in the squeeze packs. GM specifies slightly different oil viscosity ratings for the front and rear differentials for fuel mileage reasons, but 75w-90 is acceptable for both. Amsoil wasn’t any more expensive than other products on the store shelves. I did the short term membership deal and ordered 6 quarts to get free shipping. I don’t plan to change the front fluid until 30k.
 

I removed the cover and wiped the inside of the cover and the differential housing as well as I could. It took 3.5 quarts to fill it to 0.4 inches below the bottom of the fill port. No additive is needed with the proper fluid type. I have 5000 miles since the change including some off pavement driving and the G80 locker behaves normally. 

[Tyler Gibbs]

On 5/19/2026 at 10:34 PM, Another JR said:

I changed the fluid in the rear axle of my 2021 gas 3500 hd last Fall at 15k miles. I used Amsoil Severe Gear 75w-90 in the squeeze packs. GM specifies slightly different oil viscosity ratings for the front and rear differentials for fuel mileage reasons, but 75w-90 is acceptable for both. Amsoil wasn’t any more expensive than other products on the store shelves. I did the short term membership deal and ordered 6 quarts to get free shipping. I don’t plan to change the front fluid until 30k.
 

I removed the cover and wiped the inside of the cover and the differential housing as well as I could. It took 3.5 quarts to fill it to 0.4 inches below the bottom of the fill port. No additive is needed with the proper fluid type. I have 5000 miles since the change including some off pavement driving and the G80 locker behaves normally. 

This.  Amsoil SVG is top tier.  The only other fluid I would run would be HPL for the diffs.  Transfer case, any ole Dex-VI works fine.  Walmart has the Valvoline red and blue bottles.  Or you could go with Amsoil as well.

[Pryme]

I also put amsoil severe gear in my rear diff. No issues works as it should and unlike whatever dummy that under fills these diffs when new, I put the proper amount in, which is the reason I swapped out the fluid in the first place. 

[Core-Lokt]

Thanks, y'all. After reading this, I ordered the Amsoil. It's quite a bit more than Valvoline full-synthetic, but I reckon it's worth the premium.

[Another JR]

Here are pictures of the specific Amsoil product I ordered (GL-5). The price was about $17.25 per quart plus a $10 six month membership fee. Shipping was free because the order exceeded $100 for six quarts. Valvoline in my local stores was around $18. 

[Black02Silverado]

On 5/19/2026 at 8:40 PM, Core-Lokt said:

I'm going to change the fluids in both differentials and the transfer case. If anybody knows specs/capacities on these, could you post them? And is there a friction additive for the rear differential. Thanks in advance.

You can look up the capacities from AMSOIL on their look up guide.   https://www.amsoil.com/c/products/1/?zo=521390

 

Let me know and I can get you the best price.

[22SLT]

I have an SU4 front axle and an SU8 rear axle.  Which I only know for sure because I matched photos of my rear differential and then went back to the dealership for my build sheet.  Those RPOs do not appear when I scan the QR code on my door jamb. 

 

SU4 spec: ACDelco High Efficiency Gear Oil 75W-85 API GL-5, (GM part #19300457)

SU8 spec: ACDelco 75W-85 API GL-4 (GM part #19368514) 

 

Lots of warnings about using the wrong spec fluid, yellow metals and EP additives and so on.  But on the forums a lot of people say you can use Amsoil Severe Gear 75W-90, which is GL-5, no matter what differentials you have.

 

Is that specifically because the Amsoil data bulletin states "Can also be used in axles where an API GL-4 lubricant is recommended"?

 

When I get these changed I will get the correct spec fluid for each.  But logically, it seems to me that there must be millions of functioning trucks on the roads where the owner or mechanic used 75-85 or 75-90 without looking up data sheets on the internet first.

[asilverblazer]

6 hours ago, 22SLT said:

I have an SU4 front axle and an SU8 rear axle.  Which I only know for sure because I matched photos of my rear differential and then went back to the dealership for my build sheet.  Those RPOs do not appear when I scan the QR code on my door jamb. 

Visual match isn't reliable, they might be the same externally but could have different internals. (Long ago, some traction adders have different requirements)

 

Is there no longer a glove box sticker?

[22SLT]

1 minute ago, asilverblazer said:

Visual match isn't reliable, they might be the same externally but could have different internals. (Long ago, some traction adders have different requirements)

 

Is there no longer a glove box sticker?

There's a QR code on the driver's side pillar.  You scan that and it brings up your RPO codes.  But it's an incomplete list.  I ran my VIN through gmpartsgiant.com which was surprisingly accurate, and then I went back to my dealership for the full build sheet to confirm my differentials.  Also I didn't see it until now but this is the HD forum and I have a 1500.  Sorry about that.

[Another JR]

I have no first-hand knowledge about it but I have seen quite a few comments that GM selected lower viscosity gear oils to slightly improve fuel mileage. Personally, I care more about longevity and reliability than fuel mileage on this type of vehicle, and I have not read any cautions about using 75w90 in place of the 75w85 specified for the front differential, so I went with the 75w90 I have used for 35 years on other GM trucks and SUVs. 

Edited June 4 by Another JR

[Chuck FB]

7 hours ago, Another JR said:

I have no first-hand knowledge about it but I have seen quite a few comments that GM selected lower viscosity gear oils to slightly improve fuel mileage. Personally, I care more about longevity and reliability than fuel mileage on this type of vehicle, and I have not read any cautions about using 75w90 in place of the 75w85 specified for the front differential, so I went with the 75w90 I have used for 35 years on other GM trucks and SUVs. 

 

I know when I was looking around last year to see if someone handled Amsoil gear oil to buy at the retail level which of course means full price. I found an independent shop that works a lot on GM/Duramax pickups and they did not bring in the 75W-85 as they just stuck to the 75W-90 for the front and rear as a standard practice for the HD trucks which makes sense anyway for the higher hp/torque diesel and pulling heavy loads in four wheel drive. I haven't changed my diff oils yet but still plan on using the 75W-85 for the front diff as I highly doubt I will be beating on the truck and figured for my use type the bit thinner oil would be to my benefit for the winter for that slightly less drag. Very different weather here all winter vs northern Washington near the coast, that's for sure. 

[Core-Lokt]

It's raining on and off today, but I was able to change the rear oil and cover before it started. I ended up getting the Amsoil 75W85 for the front, 75W90 for the rear, and the right fluid for the transfer case. I wanted a cover with a drain, so that's pretty much the reason for the aftermarket one. I have a Roadmaster sway bar and had to fabricate 1" spacers for clearance, so that might be helpful if anyone has that on their truck and considering the cover swap. Old oil had 60,000 miles and just the slightest metal fuzz on the magnet; everything looked super nice. Fourth pic is just to highlight the spacers to accommodate the cover. Those bags sure make it easier than the old siphon guns.

[Chuck FB]

That's certainly good to hear that there was nothing alarming on the cover magnet volume wise or chunk wise of metal. I don't recall if the Banks cover holds more oil than the factor cover, what volume did you estimate that it took ?. As to the transfer case I assume you bought whatever brand you wanted of a Dexron VI automatic transmission fluid. 

 

Have you changed the automatic trans fluid at some point and actually I don't recall if your truck is the gas or diesel version as that makes the difference as to which transmission and oil type is required in 2023. The problem with my truck is that its the gas engine with the 10 speed and I've not yet attempted to see if I can drop the pan as the silly exhaust is right in the way and I do not want to touch the exhaust system on the manifold end if I can help it, the diesel does not have the Y pipe style exhaust under the pan so is not an issue to remove the pan.

[Core-Lokt]

I think the Banks cover is higher capacity, given I don’t see oil in the sight glass. They say to observe factory capacity over sight glass. I used approximately 6 1/4 pints bc I assume not being able to fully drain bags, and a tiny bit of spillage when I squeezed. Got Amsoil ATF as-indicated on their website. 
 

I have gas engine and six speed. Transmission service will happen after the transfer case and front differential (I’ll likely get another cover with drain, also). Besides the dipstick mod, idk what’s all underneath the pan.