Changing diff fluids in 2001 HD Silverado 11.5AAM diff. Gov Lock? Limited Slip? Transfer Case fluid?

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My searches about this differential seem to produce more confusion than good info, on Google. I bought an immaculate dark green metallic 2001 Silverado 2500 HD with the 8.1L big block. I've never been a truck guy but this thing just puts a big smile on my face when I drive it. The old fella who owned it most of its life took exceptional care but it doesn't look like a diff cover has been off it so I'd like to service them. GM specs a 75w90 synthetic for the rear only, regular 80w90 in the front.

My main questions are, shouldn't be an issue to use syn 75w90 in the front and rear? Did GM use a limited-slip in any of these diffs? Transfer case fluid, specs DEX3. In my experience, this is a PITA to find nowadays. Can I run Maxlife or something similar?

2001 is the first year GM used the 11.5" AAM rear. Does anyone know if this came with a Gov Lock? I'm hoping it has a limited slip and not a Gov Lock, it certainly behaves like a limited slip diff. The truck is a tire roaster and any which way you romp on it, it lights up both tires and never gives you any of the chatterings soon after if you make a turn and never any indication of a tire spinning sooner and it engaging. I did not like the govlocks in all my prior 10 bolt Yukon/Suburban/Tahoe.
 
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Hmmm! :unsure:
IDK much about Diff's or TC but, I use and love Mobil 1 75w90 LS Synthetic in everything that I've used including non LS Diff's. And even when the required gear oil is 80w90. Its already got the LS additive in it so it is a no brainer.

And too, I like Mobil 1 Synthetic ATF in older trannys(and TCs) which meets the old(er) DexronIII/Mercon spec. There are cheaper and more expensive fluids to use but this is what I like. (y)
 
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I’m sure others will chime in but for my later model GMC Z71 the differentials got 75W90 without posi additives. The rear diff is a locker not a posi.
 
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My main questions are, shouldn't be an issue to use syn 75w90 in the front and rear? Did GM use a limited-slip in any of these diffs? Transfer case fluid, specs DEX3. In my experience, this is a PITA to find nowadays. Can I run Maxlife or something similar?

75w-90 is fine in the front. Maxlife Dex 3 would work fine in the transfer case. Wal-Mart usually stocks it.

That rear axle should have a "gov lock" differential (G80 option code), which does not require a special additive. Actually, you should use a gear oil that does not contain a limit slip additive, as it will reduce the functionality of the locker. My Suburban has a gov lock differential, and the last time it was serviced by the dealer they used AC Delco 80w-90 without any limited slip additives. You can get AC Delco 75w-90 without limited slip additives, but it is somewhat difficult to find equivalent products in quart bottles (most of it is made for heavy duty trucks and comes in pails).
 
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75w-90 is fine in the front. Maxlife Dex 3 would work fine in the transfer case. Wal-Mart usually stocks it.

That rear axle should have a "gov lock" differential (G80 option code), which does not require a special additive. Actually, you should use a gear oil that does not contain a limit slip additive, as it will reduce the functionality of the locker. My Suburban has a gov lock differential, and the last time it was serviced by the dealer they used AC Delco 80w-90 without any limited slip additives. You can get AC Delco 75w-90 without limited slip additives, but it is somewhat difficult to find equivalent products in quart bottles (most of it is made for heavy duty trucks and comes in pails).
Maxlife isn't a Dex 3, technically, it's a Dex VI clone. And Redline has a product without limited slip additives, which are sold in quart bottles, if something locally cannot be found
 
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Maxlife isn't a Dex 3, technically, it's a Dex VI clone.

Thanks for pointing this out. I thought they used to offer a Maxlife Dex 3 fluid - or am I dreaming? Regardless, I see that the Maxlife ATF is now a multi-vehicle product with a lower viscosity than Valvoline's other Dex 3 fluid. It has been a long time since I have wrenched on one of these trucks, but I recall that a lot of the Duramax guys were arguing about the use of Dex VI vs. a TES-295 spec. fluid in the Allison transmission.

Since most GM dealers no longer stock Dex 3 fluid, I wonder what they use when servicing this transfer case? Might be worth a call to the local service department and see what they say.
 
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Thanks for pointing this out. I thought they used to offer a Maxlife Dex 3 fluid - or am I dreaming? Regardless, I see that the Maxlife ATF is now a multi-vehicle product with a lower viscosity than Valvoline's other Dex 3 fluid. It has been a long time since I have wrenched on one of these trucks, but I recall that a lot of the Duramax guys were arguing about the use of Dex VI vs. a TES-295 spec. fluid in the Allison transmission.

Since most GM dealers no longer stock Dex 3 fluid, I wonder what they use when servicing this transfer case? Might be worth a call to the local service department and see what they say.
It was probably Dex 3 before Dex VI came out, if I had to guess. I'm not sure if Dex VI is backwards compatible to Dex III in transfer cases. You can still buy a Dex III, but it would be unlicensed, for example, Valvoline has a Dex/Merc fluid

 
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My 2002, and my last 2003, both with 8.1's, I run Transynd or Dex/Merc in the Allison, Dex/Merc or Royal Purple Syncromax in the transfer case (just because I have a bunch of it), and whatever in the AAM 11.5. I've used 75w-90, 80w-90, and 75w-140 over the years. It has a drain plug so it's easy to swap out. The gov lock stinks (G80 code in the glovebox) but it's what comes from the factory in all of them.

The Dex/Merc is available at any farm store. It's cheap and it works just fine.
 
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When GM first started using synthetic in the pickup diffs, I remember some talk of an incompatible vent hose on the front. I think there's a revised hose with a different color. That might be worth looking into before you decide what to put in the front diff.
 

Gasbuggy

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When GM first started using synthetic in the pickup diffs, I remember some talk of an incompatible vent hose on the front. I think there's a revised hose with a different color. That might be worth looking into before you decide what to put in the front diff.

You are correct. You're supposed to upgrade the black vent to a white one. Although I found out that that is for 2002-03 models.
 

Gasbuggy

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Thanks for all the advice guys. Especially the comment about LS additive causing the gov lock to not behave properly. If true, that would explain why my last Yukon's diff was such a pain in the butt. Slow to engage, slow to disengage.
 
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