Rear Diff- mineral vs "synthetic" for hwy use

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lomez

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So, for regular highway driving (no off-road, towing, etc)....why would anyone use "synthetic" (aka highly refined mineral) gear oil? I mean, I want my rear-end to last 500,000 miles, and will use the absolute best lubrication to protect it, in order to get there; but, I just don't understand the concept behind using "synthetic", at 2-3x the cost, in a differential. Since it sees no combustion or combustion temps, I see next to no benefit....other than perhaps extended change intervals (but even that is debatable).

I'm going to dump and refill my '07 4Runner's rear pumpkin this weekend, and have always used "synthetic"....but I need to be educated on exactly how/why this makes any sense (again, other than theoretically longer ci).
confused.gif
If I'm missing something, please educate me.
 
better fuel economy due to higher VI,

more resistant to shear.

also you say "synthetic aka highly refined"

There is group IV and Group V gear oils too.

such as redline, motul etc.
 
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I run Mobil 1 75w-90 in my T4R....1997.

I changed out the rear diff, transfer case and front diff about 1+ year ago. I also changed out the AT fluid and the Power Steering fluid, to Mobil 1, at the same time for all fluids.

The only reason for the Mobil 1, was it was less than anything else at the time. Advance blocked the Valvoline Max Life, which I also happen to like, so I picked up Mobil 1 in ATF and Gear Lube using the Coupon Codes at Advance.

Both the Mobil 1 ATF and Gear Oil, after discounts, came out to a little over $5 a quart.

The fluid runs nice and smooth and I may never need to change these fluids again with the synthetic Mobil 1.
 
According to a study by Amsoil, the most important lube change in a differential is the first one - as it gets the initial break in wear particals out and greatly extends life. They did alot of UOA's (especially partical analysis) on the initial and subsequent changes to develop this conclusion.

I actually overheated a differential in a Jeep XJ once while towing. since then I only run Amsoil - and I do many thousands of miles of heavy towing every year in hot weather. (12,000+ lbs total)

in your application I would agree it might not be worth the extra cost for synthetic.
 
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Many axles call for synthetic and come with it from the factory. That's one huge reason right there to use it. Its all very vehicle specific. Upgrade to syn if cost isn't an issue but don't go mineral if the manual states syn.
 
Not a big $$ cost difference, if any savings at all after you subtract fuel saved (a comparable grade synthetic will create less friction generally). My rear diff takes 0.75 qts I believe. It's just one bottle of gear oil. I used M1 75-90 as well.
 
Shear resistance is huge.
In a diff, there is a lot of shearing going on. Same for trannys.

$10.00 more for years of use? Are you kidding me? Go full synth in geared situations.
 
differentials are one of the few places I have seen a mpg improvement over mineral. For that reason alone I favor syn. syn in the engine has never made any measurable difference however. just driveline... diff, trans, and if the front diff spins all the time I put it in there too (most awd/4wd vehicles i've seen these days do not disconnect at the hubs, so all that stuff spins...)
 
Originally Posted By: mechtech2
Shear resistance is huge.
In a diff, there is a lot of shearing going on. Same for trannys.

$10.00 more for years of use? Are you kidding me? Go full synth in geared situations.


Yea, I guess my questioning comes when using a "synthetic" that is group III......vs Group IV and V true synthetics. I know for certain there are benefits from using IV and V (particularly if you're towing and see extra heat). But III? Hmmmmm, I wonder.

After doing some research using Gear Solutions and Machinery Lubrication as sources, I am reasonably sold on IV and V (ester) oils, even for "regular highway use" (mostly due to efficiency gains). However, one thing I did notice is that polyglycol (PAG) lubricants tested to be by FAR the best. Why wouldn't they be offered (so far as I know?) for automotive gearboxes of all types? Cost?
 
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Originally Posted By: lomez
Originally Posted By: mechtech2
Shear resistance is huge.
In a diff, there is a lot of shearing going on. Same for trannys.

$10.00 more for years of use? Are you kidding me? Go full synth in geared situations.


Yea, I guess my questioning comes when using a "synthetic" that is group III......vs Group IV and V true synthetics. I know for certain there are benefits from using IV and V (particularly if you're towing and see extra heat). But III? Hmmmmm, I wonder.

After doing some research using Gear Solutions and Machinery Lubrication as sources, I am reasonably sold on IV and V (ester) oils, even for "regular highway use" (mostly due to efficiency gains). However, one thing I did notice is that polyglycol (PAG) lubricants tested to be by FAR the best. Why wouldn't they be offered (so far as I know?) for automotive gearboxes of all types? Cost?


PAG oils absorb moisture very quickly, that is why you only find it in brake fluid and air conditioning systems in a modern car.
 
Originally Posted By: lomez
So, for regular highway driving (no off-road, towing, etc)....why would anyone use "synthetic" (aka highly refined mineral) gear oil? I mean, I want my rear-end to last 500,000 miles, and will use the absolute best lubrication to protect it, in order to get there; but, I just don't understand the concept behind using "synthetic", at 2-3x the cost, in a differential. Since it sees no combustion or combustion temps, I see next to no benefit....other than perhaps extended change intervals (but even that is debatable).

I'm going to dump and refill my '07 4Runner's rear pumpkin this weekend, and have always used "synthetic"....but I need to be educated on exactly how/why this makes any sense (again, other than theoretically longer ci).
confused.gif
If I'm missing something, please educate me.


Since you are a road warrior, an industrial gear oil might suit you well and protect better long term than the typical commercial automotive gear oils. There are many conventional, blend and synthetic formulas out there. Just do some research. If you could get by with a mono-grade in your climate, that's what I'd target.
 
Originally Posted By: INDYMAC
Originally Posted By: lomez
So, for regular highway driving (no off-road, towing, etc)....why would anyone use "synthetic" (aka highly refined mineral) gear oil? I mean, I want my rear-end to last 500,000 miles, and will use the absolute best lubrication to protect it, in order to get there; but, I just don't understand the concept behind using "synthetic", at 2-3x the cost, in a differential. Since it sees no combustion or combustion temps, I see next to no benefit....other than perhaps extended change intervals (but even that is debatable).

I'm going to dump and refill my '07 4Runner's rear pumpkin this weekend, and have always used "synthetic"....but I need to be educated on exactly how/why this makes any sense (again, other than theoretically longer ci).
confused.gif
If I'm missing something, please educate me.


Since you are a road warrior, an industrial gear oil might suit you well and protect better long term than the typical commercial automotive gear oils. There are many conventional, blend and synthetic formulas out there. Just do some research. If you could get by with a mono-grade in your climate, that's what I'd target.


That's a good idea. A mono grade is less prone to shear which could extend the service life of the fluid.
As far as the conventional/syn debate a syn may likely last longer in actual service and may retain viscosity for longer.
If a person wants the best as far as gear oil I absolutely love red line shockproof gear oil. It really made an improvement in my Harley's tranny and my gears are ultra quiet in the mustang.
 
Quote:
Why wouldn't they [PAG] be offered (so far as I know?) for automotive gearboxes of all types? Cost?



We are working on it, but the oil soluble PAG base oils are expensive right now.

An alternative might be a PAO/mineral oil Blend. Contact 'salesrep' here in a PM and speak with him about Schaeffer's blended gear oils.

Quote:
PAG oils absorb moisture very quickly, that is why you only find it in brake fluid and air conditioning systems in a modern car.


Not the newer oil soluble PAG's. When discussing brake fluid and air conditioning systems you are speaking about a different set of PAG chemistries.
 
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I use synthetic because that's what the factory spec for, and couldn't find with mineral: 75w85, or 75w90.

To be honest this is a great place to use synthetic and just leave it for 2x or 3x as long as you would have with mineral.
 
Quote:
Many axles call for synthetic and come with it from the factory.
Some of these have skimped on the material and size of the assembly to save weight and initial cost. The syn oil is to try for normal life from these undersized gear assemblies.

PAO based syn gear oil will have higher film strength than petro base. If the OP doesn't load the gears heavily and doesn't need the protection in frigid winter or very hot summers, there is little reason for syn.
 
I'm a fan of synthetic gear oil, even though it costs 2x or more in my area the ACTUAL cost different isn't that huge ($6/qt for dino, $12/qt for synpower, $20/qt M1 but your only using a couple qts... well except my truck with big old sterling rear...)

But interestingly when I had a dif overhauled by DTS in Warren, MI they emphatically recommended dino oil only stating it helps transfer heat from the gear faces better. Not saying I'm sold on it, just passing the statement on.
 
Originally Posted By: buck91
I'm a fan of synthetic gear oil, even though it costs 2x or more in my area the ACTUAL cost different isn't that huge ($6/qt for dino, $12/qt for synpower, $20/qt M1 but your only using a couple qts... well except my truck with big old sterling rear...)

But interestingly when I had a dif overhauled by DTS in Warren, MI they emphatically recommended dino oil only stating it helps transfer heat from the gear faces better. Not saying I'm sold on it, just passing the statement on.


The comment about heat transfer is not correct. There is abundant data showing that the heat transfer property of PAO is about 10% better than mineral oil.
You might also consider a 50/50. PAO group III formulation which firstly reduces cost but also can provide improved performance over a full PAO blend because of better solubility. There are several around.
 
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