Mechanics step in please--gear oil front/rear differentials

May 19, 2018, 07:57 PM

I know this is going to stir a debate but the rest of the internet has me confused and overthinking.

I own a 2005 GMC Sierra 2500HD 4WD with the Duramax and Allison.

I'm going to change out the oil in both differentials. I've settled on 75-90 synthetic to put in both. My question is which brand to use. Amsoil severe gear, Mobil 1, Royal Purple, Redline....so many options.

Any help would be appreciated.

Thanks,

CT

May 19, 2018, 08:03 PM

c1steve

Mobil 1 is not rated for some diesels, so I would count that out. I just would not trust that it would be heavy duty enough for the gear mechanisms. Which brand matters little, what is more important is that you change it occasionally.

If you have limited slip in either differential, you should stick to what the factory says.

May 19, 2018, 08:15 PM

rburg

If memory serves me, Mobil 1 makes two thickness of the gear grease. One is to replace 90w and the other for like 140 or 150w. I don't think the powerplant makes a bit of difference. They use the same differentials for both. The mere fact that you care enough to change it means those will probably last forever. The worst duty is in diffs that get submerged launching boats.

Those often run at highway speeds for hours on end, then get dunked in cool or cold water. It contracts and water gets sucked past the seals.

May 19, 2018, 08:16 PM

ChasinTails

quote:
Originally posted by c1steve:
Mobil 1 is not rated for some diesels, so I would count that out. I just would not trust that it would be heavy duty enough for the gear mechanisms. Which brand matters little, what is more important is that you change it occasionally.

If you have limited slip in either differential, you should stick to what the factory says.

Thanks for the reply. According to the code list in the glovebox, my truck has the locker in the rear. Service manual calls for 75w-90 synthetic in the rear and 80w-90 in the front. There shouldn't be a reason to not use synthetic in the front, should there?

Thanks,

CT

May 19, 2018, 08:17 PM

pbslinger

Redline or Motul if you want the best. All are good.

May 19, 2018, 08:19 PM

ChasinTails

quote:
Originally posted by rburg:
If memory serves me, Mobil 1 makes two thickness of the gear grease. One is to replace 90w and the other for like 140 or 150w. I don't think the powerplant makes a bit of difference. They use the same differentials for both. The mere fact that you care enough to change it means those will probably last forever. The worst duty is in diffs that get submerged launching boats.

Those often run at highway speeds for hours on end, then get dunked in cool or cold water. It contracts and water gets sucked past the seals.

Thanks for the reply. No boats or driving through water for me. I have enough problems as it is with out boat maintenance to pay for!

CT

May 19, 2018, 08:22 PM

Shifferbrains

You should be just fine running 75w-90 synthetic in both. Brand really doesn't matter as long as it is up to manufacturers specs.

Then change it more often. I like to change my stuff on a severe duty cycle, just in case. I use the Mobil 1 without any issues.

May 19, 2018, 08:27 PM

ChasinTails

quote:
Originally posted by Shifferbrains:
You should be just fine running 75w-90 synthetic in both. Brand really doesn't matter as long as it is up to manufacturers specs.

Then change it more often. I like to change my stuff on a severe duty cycle, just in case. I use the Mobil 1 without any issues.

Thanks for the reply.

CT

May 19, 2018, 09:15 PM

ptruck

I'm partial to the AMSOIL 75-90 severe gear, because I'm an AMSOIL Dealer. If you like technical reading, AMSOIL did a white paper called "A Study of Automotive Gear Lubes" (G2457). They tested 14 different gear lubes in this study.

May 19, 2018, 10:36 PM

wrightd

Synthetic is better always. I used synthetic in one old car that didn't call for it. Be sure to add any "modifier" if specified by the manufacturer, even if the gear oil you buy says it's already rated for limited slip.

May 19, 2018, 10:46 PM

cas

How does the differential know it's a diesel?

May 19, 2018, 11:37 PM

ChasinTails

quote:
Originally posted by ptruck:
I'm partial to the AMSOIL 75-90 severe gear, because I'm an AMSOIL Dealer. If you like technical reading, AMSOIL did a white paper called "A Study of Automotive Gear Lubes" (G2457). They tested 14 different gear lubes in this study.

Email sent to the email in your profile

Thanks,

CT

May 20, 2018, 01:02 AM

Excam_Man

quote:
Originally posted by cas:
How does the differential know it's a diesel?

By listening to the sound of the engine.

May 20, 2018, 01:20 AM

tatortodd

quote:
Originally posted by c1steve:
Mobil 1 is not rated for some diesels, so I would count that out. I just would not trust that it would be heavy duty enough for the gear mechanisms. Which brand matters little, what is more important is that you change it occasionally.

If you have limited slip in either differential, you should stick to what the factory says.

Mobil 1 is for passenger cars and light duty trucks, and Mobil Delvac 1 is for heavy duty applications.

As a comparison, Mobil Delvac 1 75W-90 meets a longer list of industry specs than Amsoil Severe Gear 75W-90.

The OP should look in his owner's manual and see what spec is required (e.g. API GL-5/MT-1) and find a 75W-90 from a reputable company (Mobil Delvac 1, Amsoil Severe Gear, Valvoline SynPower, Castrol Syntec, etc)

May 20, 2018, 02:45 AM

charlie12

quote:
Originally posted by tatortodd:
quote:
Originally posted by c1steve:
Mobil 1 is not rated for some diesels, so I would count that out. I just would not trust that it would be heavy duty enough for the gear mechanisms. Which brand matters little, what is more important is that you change it occasionally.

If you have limited slip in either differential, you should stick to what the factory says.
Mobil 1 is for passenger cars and light duty trucks, and Mobil Delvac 1 is for heavy duty applications.

As a comparison, Mobil Delvac 1 75W-90 meets a longer list of industry specs than Amsoil Severe Gear 75W-90.

The OP should look in his owner's manual and see what spec is required (e.g. API GL-5/MT-1) and find a 75W-90 from a reputable company (Mobil Delvac 1, Amsoil Severe Gear, Valvoline SynPower, Castrol Syntec, etc)

It's not going in the engine

May 20, 2018, 06:37 AM

Black92LX

Amsoil is my go to for differentials and transfer cases all get done every 50k Miles.

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May 20, 2018, 07:48 AM

2012BOSS302

I use all Redline in my mustang (eng,tran,diff). The gear oil has friction modifiers in it for limited slip differentials, they sell a 4 oz bottle and I would put that in too. I would use synthetic front and rear.

https://www.redlineoil.com/

May 20, 2018, 08:18 AM

rtquig

I used Valvoline synthetic in my 2002 Chevy Duramax. I used the 75W-90, didn't tow a lot so I didn't use the 75W-140 in the rear diff. Worked fine, IIRC I didn't need friction modifiers.

May 20, 2018, 09:57 AM

henryaz

quote:
Originally posted by cas:
How does the differential know it's a diesel?

I doubt it knows it is a diesel, but it certainly knows there is a lot more torque being applied to the pinion and ring gears.

May 20, 2018, 10:15 AM

btanchors

I'm an amateur tribologist, with no professional credentials in this field, just a great interest for the past 40 years...

I would read the manual and make sure whatever you use meets those specifications, including the viscosity grade.

Of the brands you mentioned, assuming they meet the specifications, Amsoil Severe Gear, Mobil 1, and Red Line are all outstanding lubricants, among the best in the industry.

I've been using Amsoil Severe Gear for the past 10+ years with good results.