# Gear Oil Weights



## Stik208 (Oct 19, 2004)

Hello,
I am about to change my differentials and transfer case fluid in a few weeks. The rear diff is 2qts of 75w90 the front is 80w90. I plan on using Mobil 1 full synthetic in all 3 it comes in 2 verisons 75w90 and 75w140. Would It be ill advised to run the 75w90 in the front diff and Mobil 1 ATF in the transfer case which im not sure which NVG number (shift on fly w/ auto 4wd function)? Truck is a GMT900 Sierra 1500.


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## 2COR517 (Oct 23, 2008)

Always a wise maintenance project Jason, often overlooked until something starts making noise.

Mobil1 75W-90 front and rear diffs. I think it's on sale at AutoZone right now, not sure. 

For the NP246 transfer case with Auto4wd, the only fluid you can use is GM's own blue juice AutoTrac fluid. You will need two quarts. Anything else and you will be rebuilding your transfer case soon. Given that M1 ATF is pushing ten bucks per quart, the GM stuff won't be much more $$.


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## theholycow (Nov 29, 2002)

Don't use synthetic where it's not specified. I can't remember which differentials exactly, but some have a gasket that's sensitive to synthetic.

As he said, do use *only* AutoTrak II fluid in the transfer case. At $8/quart from the dealer it's priced just fine and it's the only right fluid to use.


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## Newdude (Dec 4, 2006)

GMT-900 are Magna transfer cases now (Magna picked up NVG a few years ago). AutoTrak is what you want for auto 4x4 like others stated.


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## 70monte (Feb 5, 2008)

Take this for what it is worth and most people probably won't have problems but I put Mobil 1 75W-90 synthetic diff fluid in both diffs of my 98 K1500 years ago when the truck only had 20,000 miles on it. At around 55,000 miles I developed a slight whine that was coming from the rear of the truck. I didn't think too much about it until around 76,000 when it had gotten a lot louder. I replaced both U-joints which did not fix the problem. I took it to a driveline shop and they diagnosed bad carrior and pinion bearings in the rear end. This was on a truck that only hauled a 5x10 utility trailer a few times and never anything heavy in the bed. 

I currently have some noise coming from the front diff somewhere which will have to be addressed soon. This truck only has 121,000 miles on it and I have taken very good care of it. I had also changed the Mobil 1 diff fluid twice during the time I used it.

Later I was told that I should have kept the diff oil conventional since my diffs were not speced for synthetic. I have not used synthetic diff fluid since.

I believe the rearend on your truck uses synthetic from the factory so you should be good on that. I would personally stick to what the owner's manual says to use for the front but that is just me.

Wayne


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## chevboy167 (Dec 7, 2011)

most manuals state 80-90w in front diff unless poor front diff engagement in cold weather. then ok to use 75-90w synthetic in front. however, i did just that and had drivers side axle stub seal leak all the time. even after i replaced it 5 times with an OEM GM seal. my plant maint guru (owns his own huge mechanic shop & heavy truck repair!) said the front diff seals dont like the synthetic oil for some reason. they will always leak 3 months down the road. he rebuilt my front diff & filled with the 80-90. all is good as of 4 months so far. rear diff gets the 75-90 syn at all times.


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## Stik208 (Oct 19, 2004)

I have been doing some research on this whole thing, today I bought the Autotrak 2 for $8.95 a qt. I read on a Cadillac forum, a fellow did his front diff of this EXT with the 7590 M1 and had a problem. Further reading he went to 75w140 M1 and has had no issues and no mention of leaks. I would like the keep everything synthetic so would I am asking 75w140 (syn) vs 80w90 (conv) for the front diff?


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## theholycow (Nov 29, 2002)

What problem did he have? Can you post a link to the thread? I'm interested.

As for your truck, I personally would stick with exactly what the manual recommends.


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## NICHOLS LANDSCA (Sep 15, 2007)

70monte;1447483 said:


> Take this for what it is worth and most people probably won't have problems but I put Mobil 1 75W-90 synthetic diff fluid in both diffs of my 98 K1500 years ago when the truck only had 20,000 miles on it. At around 55,000 miles I developed a slight whine that was coming from the rear of the truck. I didn't think too much about it until around 76,000 when it had gotten a lot louder. I replaced both U-joints which did not fix the problem. I took it to a driveline shop and they diagnosed bad carrior and pinion bearings in the rear end. This was on a truck that only hauled a 5x10 utility trailer a few times and never anything heavy in the bed.
> 
> I currently have some noise coming from the front diff somewhere which will have to be addressed soon. This truck only has 121,000 miles on it and I have taken very good care of it. I had also changed the Mobil 1 diff fluid twice during the time I used it.
> 
> ...


Not picking on you but just using your situation as an example. The rear end went 56,000 miles on the "bad"oil. It wasn't the oil that caused the issue, the 10 bolts in those trucks are NOT strong enough for the application. They should have put the axles out of the trucks in the cars and used the ones out of the cars as anchors, a broken 10 bolt is nothing new. I always find it funny how X product caused an issue, if that were the case they would be out of business.
To the OP run the 80-90 front and rear and smurf juice in the t-case, for what it's worth I know of a bunch of trucks that had synthetic ATF dumped in them and never had an issue


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## 70monte (Feb 5, 2008)

I didn't start using Mobil 1 diff fluid until I had 20,000 miles on the truck. At around 40,000 miles I changed both diffs again with Mobil 1 so I didn't run this fluid for the entire 56,000 miles.

At around 75,000 miles when the rearend was repaired, conventional fluid went back in and I have not had any problems since with 121,000 on the truck. 

I know the 10 bolt rears are weak for truck applications and I'm not saying that running Mobil 1 definately caused my problems but I decided to change back to conventional fluid and don't regret doing so.

Wayne


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## Stik208 (Oct 19, 2004)

theholycow;1451326 said:


> What problem did he have? Can you post a link to the thread? I'm interested.
> 
> As for your truck, I personally would stick with exactly what the manual recommends.


http://www.cadillacforums.com/forum...66-front-diff-transfer-case-how-change-2.html - post 22 I cannot get to his thread about the whining from the front since I am not a member there.


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## 2COR517 (Oct 23, 2008)

I swapped the diffs in my 96 Tahoe to M1 75w-90 last year. 200k on the clock. No ill effects.....


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## Stik208 (Oct 19, 2004)

2COR517;1451810 said:


> I swapped the diffs in my 96 Tahoe to M1 75w-90 last year. 200k on the clock. No ill effects.....


Thats what I am looking to here, I was at the dealer getting the Autotrac yesterday and you wont believe the price of Delco 75w90 syn gear lube..........$35 a quart   :laughing:

I have a set of regular hand tools (Craftsman 200 piece "mechanics" set) is this a project I could tackle myself? I always do my own oil, I have access to ramps but not a lift. I also have a rear diff without a drain plug so I am going to grab a new gasket as well. I also dont know if I have a g80 locker by my vin 2gtek13mx71525652?


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## theholycow (Nov 29, 2002)

Stik208;1451877 said:


> I also dont know if I have a g80 locker by my vin 2gtek13mx71525652?


Look at the RPO codes on the label in your glovebox. "G80" is actually the RPO code for upgraded limited slip/locking differential, not a specific model of differential, so if you have a G80 then it will be printed exactly as "G80" on the label.

Feed your other codes into an RPO decoder (google for one) to find out what other equipment you have, or take your VIN to a dealer for a GMVIS report for a complete list.


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## 2COR517 (Oct 23, 2008)

Stik208;1451877 said:


> Thats what I am looking to here, I was at the dealer getting the Autotrac yesterday and you wont believe the price of Delco 75w90 syn gear lube..........$35 a quart   :laughing:
> 
> I have a set of regular hand tools (Craftsman 200 piece "mechanics" set) is this a project I could tackle myself? I always do my own oil, I have access to ramps but not a lift. I also have a rear diff without a drain plug so I am going to grab a new gasket as well. I also dont know if I have a g80 locker by my vin 2gtek13mx71525652?


Should be an easy project for you. I haven't worked on anything as new as your truck, but it should be generally the same. A good rule of the thumb on these project is to make sure you can get the fill plug out BEFORE the drain plug (or pulling the cover on the rear). You don't want to get yourself in a situation where the gearbox is empty and you can't fill it back up.

The front case should be real easy, nice big hex head bolts for the drain and fill. If you have a skid plate, it's probably worth the time to remove it for better access. You will also want a couple feet of fuel hose to fill the front diff.

The transfer case may be allen heads for the drain and fill. It's probably 10mm, don't use a 3/8 it's too small. My older Tahoe has big hex heads. Had to buy a socket just for them, I think it's 30mm.

The rear fill is usually a 3/8 square, just use an extension on your ratchet. Many guys just use RTV on the rear diff cover, I prefer a gasket with a bit of high tack


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## thermo (Apr 16, 2011)

Shouldnt be to bad. As 2cor517, take the fill plugs out before the drain plugs. I use a siphon pump for adding oil/fluid to my diffs/transfer case/trannys to make it easier. I dont know why GM stopped putting drain plugs on their 10 bolt axles.


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## Stik208 (Oct 19, 2004)

I did the transfer case today, it appeared to have ATF in as the old fluid was reddish/brownish/black no burned smell which I thought was a plus. Filled it with just under 2 qts of Autotrack 2.


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## B&B (Nov 4, 2006)

Does your truck have an "Auto" position on the dial?


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## Stik208 (Oct 19, 2004)

B&B;1452948 said:


> Does your truck have an "Auto" position on the dial?


It does, I never use it.


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## B&B (Nov 4, 2006)

Then you did good by switching to the smurf fluid. The old fluid appeared red because it was likely Dex VI. The AutoTrak II is the better choice of the two.


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