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Discussion Starter #1 (Edited by Moderator)
I'm the original owner of a 2007 2500HD Classic 4WD with 65K miles. All other maintenance has been kept up to date, but I haven't replaced the differential and transfer case oil. Should this be done soon? If so, should I use a synthetic for longevity?
 

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I would use synthetic, it likely won't make a real difference what you select.


Owners manual will indicate what fluids to use. If it says to use a DexIII fluid in the transfer case, it's suggested that you actually find a DexIII fluid and use it (vs using the now more commonly available DexVI that's considered backwards compatible), as there were some problems with seals leaking when using DexVI, that didn't with DexIII.


And if the rear diff is a G80, GM's recommendation is to not add a limited slip differential modifier (LSD) to the fluid, while Eaton (who makes the G80) says to add some. Some use stuff with an LSD already in it, and some people just use plain gear oil (and some of these people notice their diff doesn't lock up as nice as before, and add a bit of LSD and it's good again).
 

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For the G80, my understanding is that GM says not to add friction modifier because their "grapejuice" already has some in it, so don't add more. So, it needs some, but not too much. The easiest route is to use the genuine GM gear oil (very expensive), or go something like Mobil 1 Synthetic 75-90 LSD gear oil (which I've used for over 10 years without any issues). Use it in the front diff, too.

I don't know what GM said for the transfer case in 2007, but if you manual says Dex-III, then like davester said, find some. Again, Mobil 1 has a transmission fluid "for GM 2006 and older" that means Dex-III, but they might not be able to say Dex-III because the licensing was retired. Last time I looked, some still say Dex-III, or maybe compatible with Dex-III, like some Walmart products. Or maybe Valvoline Maxlife. Like all above, synthetic is a nice touch for these fluids.
 

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I'm the original owner of a 2007 2500HD Classic 4WD with 65K miles. All other maintenance has been kept up to date, but I haven't replaced the differential and transfer case oil. Should this be done soon? If so, should I use a synthetic for longevity?
First welcome to DP

I’m still the Original Owner of our 07 Classic for another month and I have all the Owners Manuals including Diesel Supplement Manual.
6-14 Page 550 of your Owners Manual
Front & Rear Axle
SAE 75W-90 Synthetic
Axle Lubricant
(GM Part No. U.S. 12378261, in Canada 10953455) meeting GM Specification 9986115.

Yes, I would use Synthetic of your choice since it recommended.

This might help
https://www.dieselplace.com/forum/71-information-place/19-do-yourself-useful-articles-product-reviews/198162-how-change-your-fluids-front-rear-diff-transfer-case-thru-speed-sensor-pics.html
As for the transfer case I filled through the top fill not the speed sensor that some recommended. I figured it wasn’t necessary.

There is a debate on what oil to use in your transfer case on here. If you want me to find it I can or you can do a search
 
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