Differential and Transfer Case fluid change ?'s

[reardiff]

Going to change the differential and transfer case fluids this weekend and had two questions I couldn't seem to find answers on.

 

1. How full to you fill them? I've read to fill until it starts to run out and then some say 3/8 inch below the fill hole?  If the later, how would you know when it's 3/8 inch below, that's a very specific measurement?

 

2. Since there's no rear drain plug, Is the cover gasket reusable or do I need a new one?

 

3. For the transfer case is anything Dexron VI equivalent ok, or should we be using the new HP fluid that goes in the 8 speed transmissions 

 

Completely unrelated, but I just put on a 1.5 inch level and some 275/60/20 KO2's the other day and looks great.  For those interested I left the stamped steel control arm and with the level have no rubbing, even clears the mud flaps (barely).  Made a huge difference in the appearance of the truck and couldn't be happier. I'll try to snap a few pics later.

[CamGTP]

I fill them up until it comes out the fill hole, never had a problem.

 

The transfer case uses just transmission fluid, don't run anything else other than what it says too. The new HP Fluid won't be what you want.

 

I don't know if the rear is a gasket or just RTV, often it's just RTV. Get a new gasket or use RTV again.

 

I have a fluid pump so sucking the fluid out when it's hot is easier and less of a mess.

[btj_z71]

2 hours ago, reardiff said:

Going to change the differential and transfer case fluids this weekend and had two questions I couldn't seem to find answers on.

 

 

1. How full to you fill them? I've read to fill until it starts to run out and then some say 3/8 inch below the fill hole?  If the later, how would you know when it's 3/8 inch below, that's a very specific measurement?

 

2. Since there's no rear drain plug, Is the cover gasket reusable or do I need a new one?

 

3. For the transfer case is anything Dexron VI equivalent ok, or should we be using the new HP fluid that goes in the 8 speed transmissions 

 

Completely unrelated, but I just put on a 1.5 inch level and some 275/60/20 KO2's the other day and looks great.  For those interested I left the stamped steel control arm and with the level have no rubbing, even clears the mud flaps (barely).  Made a huge difference in the appearance of the truck and couldn't be happier. I'll try to snap a few pics later.

Yep - just fill rear diff until the fluid trickles out.  You should do the front diff too.  That one is generally a lot worse than the rear.

 

I suggest using Amsoil fluids. Best you can get.

 

Regarding the gasket, yes you can re use it as long as it's not damaged and you are careful taking off the diff cover.

 

One important tip, on the front diff and transfer case, make sure you remove the fill bolt before the drain bolt.  This step makes sure you can refill after you drain and don't end up in a bad spot.

[davester]

Do you have an "old-school" manual transfer case, or do you have one with an "Auto" or "AWD" mode?

 

The ones with Auto or AWD need a different fluid than the manual TC's.

[reardiff]

Thanks - I am using Amsoil fluids but didn't mention that on purpose so it didn't get into the debate of one fluid vs the other. Seems to be a sensitive subject around here ; )   

 

It does have an 'Auto' option on the 4 wheel drive dial, but pretty sure it takes regular trans fluid, just wasn't sure if it needed the same HP fluid as the transmission, so thanks for the clarity there.  And good call on the removing fill plug BEFORE the drain plug. Wouldn't have thought of that until it's too late. 

 

So all three just fill until it starts seeps out? I have a Haynes manual on the way so I'll see what it says when it arrives as well. As always appreciate the replies.  

[midwestdenaliguy]

17 hours ago, reardiff said:

Going to change the differential and transfer case fluids this weekend and had two questions I couldn't seem to find answers on.

 

1. How full to you fill them? I've read to fill until it starts to run out and then some say 3/8 inch below the fill hole?  If the later, how would you know when it's 3/8 inch below, that's a very specific measurement?

 

2. Since there's no rear drain plug, Is the cover gasket reusable or do I need a new one?

 

3. For the transfer case is anything Dexron VI equivalent ok, or should we be using the new HP fluid that goes in the 8 speed transmissions 

 

Completely unrelated, but I just put on a 1.5 inch level and some 275/60/20 KO2's the other day and looks great.  For those interested I left the stamped steel control arm and with the level have no rubbing, even clears the mud flaps (barely).  Made a huge difference in the appearance of the truck and couldn't be happier. I'll try to snap a few pics later.

1. I fill until it seeps out, never had an issue on multiple vehicles doing it this way.

2. Looks like you've got a '17 Sierra with the 6.2, so it should have a reusable gasket. Personally, I don't reuse gaskets since you can buy a new one for ~$25-30. It's cheap insurance in my mind, but to each their own. You can also take out the old gasket and use RTV if you want to go that route. Pros and cons to each, RTV is cheap but can be messy to clean when you change fluids in the future - also need to carefully apply a consistent bead so it seals properly. Gaskets are quick and easy, but cost more. Up to you what's more important.

3. Here's what I personally use on my fluids, not pushing anything just letting you know what I go with: 5 qts total of AMSoil Severe Gear 75w-90 to cover front and rear diff's and 2qts of AMSoil ATL transmission fluid for the T-case. Front diff takes just over 1 qt (3.2 pints) and rear diff takes just over 2 qts (the 8.6 rear is 4.2 pints, 9.5 and 9.7 rear takes 5.5 pints).

 

Good luck with the fluid change. I'd recommend a fluid pump if you buy the hard plastic bottles of fluid, but there are also easy squeeze pouch bags if you look for them and don't want to buy a fluid pump.

[Black02Silverado]

24 minutes ago, midwestdenaliguy said:

1. I fill until it seeps out, never had an issue on multiple vehicles doing it this way.

2. Looks like you've got a '17 Sierra with the 6.2, so it should have a reusable gasket. Personally, I don't reuse gaskets since you can buy a new one for ~$25-30. It's cheap insurance in my mind, but to each their own. You can also take out the old gasket and use RTV if you want to go that route. Pros and cons to each, RTV is cheap but can be messy to clean when you change fluids in the future - also need to carefully apply a consistent bead so it seals properly. Gaskets are quick and easy, but cost more. Up to you what's more important.

3. Here's what I personally use on my fluids, not pushing anything just letting you know what I go with: 5 qts total of AMSoil Severe Gear 75w-90 to cover front and rear diff's and 2qts of AMSoil ATL transmission fluid for the T-case. Front diff takes just over 1 qt (3.2 pints) and rear diff takes just over 2 qts (the 8.6 rear is 4.2 pints, 9.5 and 9.7 rear takes 5.5 pints).

 

Good luck with the fluid change. I'd recommend a fluid pump if you buy the hard plastic bottles of fluid, but there are also easy squeeze pouch bags if you look for them and don't want to buy a fluid pump.

Yes, the squeeze pack is the way to go.  The ATL is good for the T case. 

 

 

[reardiff]

Yep, that's what I got 5 of the severe gear and 2 ATF's.  This will be the first time I've used the squeeze packs but they look SOOO much easier to manage than the hard plastic bottles and a pump. I think I'm going to try to be gentle with the gasket and reuse it. I'm at just over 30k, curious to see the condition of the fluids. 

[Black02Silverado]

34 minutes ago, reardiff said:

Yep, that's what I got 5 of the severe gear and 2 ATF's.  This will be the first time I've used the squeeze packs but they look SOOO much easier to manage than the hard plastic bottles and a pump. I think I'm going to try to be gentle with the gasket and reuse it. I'm at just over 30k, curious to see the condition of the fluids. 

I reused the gasket on our 2016 Suburban and all is good.  No issues.  Just be careful taking it off.

[newdude]

19 hours ago, reardiff said:

Going to change the differential and transfer case fluids this weekend and had two questions I couldn't seem to find answers on.

 

1. How full to you fill them? I've read to fill until it starts to run out and then some say 3/8 inch below the fill hole?  If the later, how would you know when it's 3/8 inch below, that's a very specific measurement?

 

Fill it till it runs out.  Let it trickle to about a drop or nothing coming out.

 

2. Since there's no rear drain plug, Is the cover gasket reusable or do I need a new one?

 

Gasket is re-useable unless damaged.

 

3. For the transfer case is anything Dexron VI equivalent ok, or should we be using the new HP fluid that goes in the 8 speed transmissions 

 

Dexron 6 only for the transfer case.

 

Completely unrelated, but I just put on a 1.5 inch level and some 275/60/20 KO2's the other day and looks great.  For those interested I left the stamped steel control arm and with the level have no rubbing, even clears the mud flaps (barely).  Made a huge difference in the appearance of the truck and couldn't be happier. I'll try to snap a few pics later.

 

[rjpoog1989]

I reused my gaskets on the rear and transmission.  Filled diffs and transfer case till it ran out.  Dex6 in transfer case.  75w-90 in diffs.  

[Jamessm]

On 8/17/2020 at 8:17 PM, btj_z71 said:

Yep - just fill rear diff until the fluid trickles out.  You should do the front diff too.  That one is generally a lot worse than the rear.

 

I suggest using Amsoil fluids. Best you can get.

 

Regarding the gasket, yes you can re use it as long as it's not damaged and you are careful taking off the diff cover.

 

One important tip, on the front diff and transfer case, make sure you remove the fill bolt before the drain bolt.  This step makes sure you can refill after you drain and don't end up in a bad spot.

Also you should remove the fill plug first to make sure your transmission seal  isn’t bad. When you remove fill plug, if you see excess fluid coming out of fill plug, it’s a sign that your transmission seal is leaking

[Z45]

On 2/15/2026 at 9:55 PM, Jamessm said:

Also you should remove the fill plug first to make sure your transmission seal  isn’t bad. When you remove fill plug, if you see excess fluid coming out of fill plug, it’s a sign that your transmission seal is leaking

Good idea to always remove or at least loosen the fill plugs. Nothing worse than draining it and then can't get the fill plug out?  Its happens. 

[Jay Ratliff]

On 2/17/2026 at 10:17 AM, Z45 said:

Good idea to always remove or at least loosen the fill plugs. Nothing worse than draining it and then can't get the fill plug out?  Its happens. 

 

Yes, always remove the fill plug first, for this reason.