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#2760430 - 10/02/12 05:36 AM Gear Oil: Synthetic vs Conventional
JDMcQ Offline


Registered: 12/12/11
Posts: 3
Loc: Pennsylvania
In a discussion about maintenance on my 2005 Jeep Liberty CRD, a long standing member of a forum insists that a conventional gear oil is the way to go in a differential. When pressed for reasons, he cited that conventionals last longer in diffs and Currie (the manufacturer of his axles) will void their warranty if synthetics are used.

What are the differences between the performance and protection of synthetics and conventional gear oils? Which is better or worse and why?

Please help me understand.

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#2760462 - 10/02/12 07:12 AM Re: Gear Oil: Synthetic vs Conventional [Re: JDMcQ]
Zaedock Offline


Registered: 09/10/05
Posts: 3843
Loc: Massachusetts
If he's running Currie's in a Libby, then he probably wheels quite a bit. In that case, running syn would be more expensive due to periodic changes from wheelin'. Personally, I run conventional 80W90 in my axles (and I run my rig hard) and know that I won't be spilling $10/qt gear oil if I have to open up the diff on the trail. I also know guys who run the famed King of The Hammers race with conventional gear oils.

Syn has a more uniform structure than dyno and can typically offer a slight economy benefit, but I have never seen a longevity comparison between the two. Millions of axles run many, many miles on conventional and in my experience there has been no performance enhancement from using syn over conventional that I have noticed. I think it's splitting hairs in your buddy's application.
_________________________
2012 F150 Ex-cab 5.0L/2004 Malibu Maxx LT
1992 YJ "The Heep"/1994 Olds Cutlass Ciera 3.1/auto
1975 Ford Bronco/1959 Willys CJ5/20XX Custom rock crawler

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#2760466 - 10/02/12 07:18 AM Re: Gear Oil: Synthetic vs Conventional [Re: JDMcQ]
meep Offline


Registered: 02/20/07
Posts: 2520
Loc: Southeast
i have found in every rwd vehicle I've owned that syn driveline fluids gain me .5 mpg or so, with the possible exception of the tundra I have now. I have not captured any data outside of noise that suggests it has helped here. but in 2 jeeps, a pathfinder, a miata, and an awd subaru, not to mention other friend's vehicles I've worked on, I've seen a difference.

Longevity? hard to say. I usually run syn lube 50k. conventional 30k.
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2006 Tundra 2wd
2002 MDX (wifey!)
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#2760472 - 10/02/12 07:37 AM Re: Gear Oil: Synthetic vs Conventional [Re: JDMcQ]
Bottom_Feeder Offline


Registered: 12/07/08
Posts: 1476
Loc: Occupied Virginia
I thought Currie doesn't want their customers to use synthetic lube due to the type of LSDs they use in their axles. Just an assumption.

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#2760485 - 10/02/12 07:52 AM Re: Gear Oil: Synthetic vs Conventional [Re: JDMcQ]
JDMcQ Offline


Registered: 12/12/11
Posts: 3
Loc: Pennsylvania
Thanks guys. I think his main reason is the warranty from the axle manufacturer. I, and most people, do not run custom axles. It just seems to me that the positives of the synthetic (longer drain intervals, higher efficiency, better lubrication) would outweigh the negatives, whatever they might be.

If I were to tear open a $8000 axle on the trail, the last thing I would worry about would be $50 worth of gear oil.

Currie will install just about any type of traction control diff requested, rarely though would someone request a clutch type LSD. More ofen a selectable locker or gear type LSD, ie Eaton Truetrac.

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#2760493 - 10/02/12 08:03 AM Re: Gear Oil: Synthetic vs Conventional [Re: JDMcQ]
Zaedock Offline


Registered: 09/10/05
Posts: 3843
Loc: Massachusetts
I would question the sanity of anyone who would buy an $8K axle for a trail rig.

I could build an entire buggy on one tons with that budget.
_________________________
2012 F150 Ex-cab 5.0L/2004 Malibu Maxx LT
1992 YJ "The Heep"/1994 Olds Cutlass Ciera 3.1/auto
1975 Ford Bronco/1959 Willys CJ5/20XX Custom rock crawler

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#2760505 - 10/02/12 08:22 AM Re: Gear Oil: Synthetic vs Conventional [Re: JDMcQ]
JDMcQ Offline


Registered: 12/12/11
Posts: 3
Loc: Pennsylvania
8000 was a guess. I think another friend got a quote for 9-10 for both axles.

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#2760548 - 10/02/12 09:36 AM Re: Gear Oil: Synthetic vs Conventional [Re: JDMcQ]
95busa Offline


Registered: 10/15/05
Posts: 1126
Loc: Tennessee
Some OEMs require synthetic, for instance Dodge in their late model 1500s and my '00 F150 require 75-140 syn. When I got my 4.56s installed my diff was making noise. When I swapped the break in oil (probably garbage oil) for 75-140 syn it got much quieter. That being said I am not scared to run whatever meets the spec in my rides. My Grand Marq has a frankenbrew of 2/3qt Amsoil 80w-90 and the rest supertech 80w-90.
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#2760582 - 10/02/12 10:13 AM Re: Gear Oil: Synthetic vs Conventional [Re: JDMcQ]
Tempest Offline


Registered: 03/21/06
Posts: 10468
Loc: Las Vegas NV
I am not familiar with Currie, but I see no reason not to run syn in any dif. Conventional dif. oil will need to add Viscosity Index Improver (VII-polymer) to get to the same low temp spec as a syn will with no VII. This is important because VII does not maintain the same "film strength" as oil does. This means that a syn without VII will provide better protection in the long run, and last longer because VII eventually breaks down.

I maintain a stock 02 Liberty and have had syn gear oil for several years now without issue. Like others have said, if you are going into water and having to change it out all the time, then going with a cheaper conventional oil would make sense.
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#2760676 - 10/02/12 12:05 PM Re: Gear Oil: Synthetic vs Conventional [Re: JDMcQ]
mechtech2 Offline


Registered: 09/05/06
Posts: 19479
Loc: Chicago Area
For differentials, transfer cases, and auto or manual trannys, I like full synthetic fluids. Their infrequent changing and possible hard use makes this a no brainer to me.

For engines, full synthetic for turbos or long OCIs.

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#2761074 - 10/02/12 06:06 PM Re: Gear Oil: Synthetic vs Conventional [Re: JDMcQ]
WishIhadatruck Offline


Registered: 05/09/06
Posts: 1295
Loc: Grand Rapids, MI
My wagon has a clutch type limited slip and it is pretty much religion on the Impala SS forum that you CAN'T use synthetic in the rear diff. with the OEM type limited slip (my wagon is the same under the skin as an Impala SS). Many have tried and complained. I really wanted to use synthetic since I tow a trailer. I noticed than many of those that tried and complained used Mobil 1, probably because Mobil 1 is readily available, popular, and well respected. I did notice a few people had tried Amsoil with no complaints, so that is what I tried. Pablo suggested a gear oil for me to try and said to use the Amsoil limited slip additive. I have had it in there for a few years now with no issues and lots of trailer towing. It might be time to change it out.
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#2761148 - 10/02/12 07:19 PM Re: Gear Oil: Synthetic vs Conventional [Re: JDMcQ]
bigjl Offline


Registered: 09/06/12
Posts: 1710
Loc: London, England
I am going to change all the diff, axle and transfer box oils to synthetic, probably Severe Gear.

I don't take it off road, but I wouldn't use cheaper fluids on the basis that they may leak out after some form of damage.

The point is they are better, simple as that. Anything manmade and designed to perform a task it is highly likely to be better than something naturally occurring even if it is refined.

Just my opinion.
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#2761162 - 10/02/12 07:28 PM Re: Gear Oil: Synthetic vs Conventional [Re: JDMcQ]
PandaBear Offline


Registered: 08/05/02
Posts: 12525
Loc: Silicon Valley
If it is a diff that needs to go into water a lot, conventional with frequent change. If it is a road only vehicle then definitely synthetic.
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#2762764 - 10/04/12 12:16 PM Re: Gear Oil: Synthetic vs Conventional [Re: PandaBear]
mechtech2 Offline


Registered: 09/05/06
Posts: 19479
Loc: Chicago Area
Originally Posted By: PandaBear
If it is a diff that needs to go into water a lot, conventional with frequent change. If it is a road only vehicle then definitely synthetic.


This is a good perspective to start from.

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#2765930 - 10/07/12 06:36 PM Re: Gear Oil: Synthetic vs Conventional [Re: WishIhadatruck]
zray Offline


Registered: 12/11/11
Posts: 45
Loc: NE Oklahoma
Originally Posted By: WishIhadatruck
My wagon has a clutch type limited slip and it is pretty much religion on the Impala SS forum that you CAN'T use synthetic in the rear diff. with the OEM type limited slip (my wagon is the same under the skin as an Impala SS). ........"


I read the same nonsense on the Mustang forums re the usage of synthetics in the engine, transmission, and rear end.

I put over 50,000 miles on a '66 Shelby GT350 over a 10 year period. A lot of these miles were in the severe duty category. Many high speed endurance runs of over 120 mph for 50 to 100 miles at a time, etc. I used Mobil 1 15w-50 in the engine and Mobil 1 75w-90 or 75w-140 in the transmission and differential (limited slip traction loc). Before I sold the car a complete driveline inspection was performed. There was zero measurable wear in the engine. Same for the transmission & rear end,

99% of all negative reports on the use of synthetics are anecdotal fearful apocalyptic ramblings that have no basis in fact.

If less wear is your goal, then you owe it to your engine, and the rest of the drivetrain as well, to use a high quality synthetic oil.

Z.
_________________________
'66 Galaxie
'65 HiPo Mustang
'66 GT350 ('01 to 2012) driven over 50,000 miles w/ Mobil 1 15w-50

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