Dino or Synthetic in Differentials

[MolaKule]

Editor Timer got me again:

Originally Posted By: from Mola's 7/02 post


CONVECTIVE Heat Transfer:

H = F.rho.c(To-Ti),
where H is heat energy in Joules, F is volume flow in cubic meters/s,
c is heat capacity in Joules/kg.C, and temps in C. The c for synthetic oil is 2000 J/kg.C and c for dino is 1780 J/kg.C. I assumed a flow rate was 1/4 liter per second, To is temp out of a journal bearing = 100 C, and Ti was oil temp into bearing = 80C, representing a temp rise of 20C, which is a rule of thumb.

Hs = convective energy transfer in Joules for synthetic = 10.65 J
Hd = convective energy transfer in Joules for dino (mineral oil) = 9.3 J.

Therefore, a PAO majority synthetic oil is 13% more efficient at convective heat transfer.

The "rho" factor (oil convection heat transfer) is the oil's density, measured in kg/cubic meters, and since both oils were so close in density, I used 1.065 kg/cubic meter.


These are heat transfer formulas from thermodynamics and the constants I used for "c" and "k" were from Michael J. Neal's, The Handbook of Tribiology.

 

[DoubleWasp]

My Navigator has "Synth Oil Only" cast into the pumpkin, so that's what I use.

 

[BrianF]

I run synthetic in my vehicles. I do not believe that for my level of use/abuse that the syn would extend the life of the diff beyond what I would use during normal temps. The main component in my decision is solely cold flow properties. I use my vehicles year round and want a splash lubricated part to have a very free flowing oil in it.

Sometimes we tow in the dead of winter from a cold start and I want the gear oil to flow at as low of a temp as possible.

 

[tig1]

It's my understanding that most all semi diffs use synthetic gear oil for cooler operation and longer life.

 

[Bgallagher]

Originally Posted By: Jimzz
Diffs don't run that hot to begin with.
There are some that have had issues and they call for syn oil. Nissan tried to use regular oil in their titans but had problem so they switched to a 75w140 syn.


My terrain comes standard now with 75w-90 synthetic

 

[stchman]

A synthetic gear lube is always going to be better than a conventional gear lube.

 

[buck91]

Ford is running synthetic OEM axle fluid...

 

[Doug Hillary]

Hi,
totegoat - IME with heavy Trucks, synthetic transmission and differential lubricants definitely run cooler. In some cases this can be as much as 20-25C depending on the application

 

[tdi-rick]

Some mineral based gear lubes can outperform average syns thanks to exceptional additive packs.
As has often been stated by those that know (Molakule, Bruce381, Terry Dyson, etc) it's the overall formulation that counts.

Jim Allen detailed the results comparing good syn diff oils with very good mineral lubes several times on this board years ago when logging diff temps in various vehicles.

 

[Doug Hillary]

Hi,
totegoat - Eaton Corp released a document (TC-019-R2) on this subject in 8/93 after a decade of development, field testing and Warranty research. They firmly endorsed synthetic lubricants in their products - for (briefly) the following reasons;

1 Extend Warranty for up to 7 years or 750 000 miles
2 "Because synthetic lubricants run cooler than mineral-based lubricants...."
3 Savings on fuel ranging from 1% to 5% ("...assume at least 1%")
4 Extended OCIs
5 Longer service life of seals and bearings

And etc.

I was part of this and I can confirm the document. Resulting, I used lifetime component OCIs on my Road Train (120t) rated Prime Movers with 18 speed RR RTLO 16718B transmissions. I never suffered lubricant related failures of any type.

Oil temperatures over many millions of kms in ambient temperatures ranging from -5C to 50C averaged;

Gearbox 84C
Tandem front diff 90C
Tandem rear diff 94C

"Normal" allowable temperature up to 120C

MB and ZF of course have similar experiences dating back many decades. The ZF Ecosplit multispeed being a good example

Modern formulation mineral differential lubricants are excellent products - as in the past however IMO modern synthetic differential lubricants still offer relevant advantages in many applications

 

[Sunnyinhollister]

Molakule,

Thanks for the info on thermal conductivity. On the Jeep boards it is taken as gospel that synthetic gear lube has LESS thermal conductance than mineral oil and the gears run hotter. Currie will actually void your warranty if you run a synthetic gear oil. Apparently there was a rash of failures and synthetic was blamed.

 

[mjoekingz28]

Mr DH,

Did you document how much quantity of oil you added over the distance?


In other words, how much fluid did the drive use?
Any consumption?

 

[Doug Hillary]

Hi,
mjoekingZ28 - Virtually no top ups were made during the OCI cycle unless repair work was done. In my case this to all affect was zero as only leaking seals would be the reason and on the synthetic lubricants

The diffs hold around 20 litres each
The gearbox holds about 13 litres

Eaton found that on Fleet experience mineral lubricants seals had a failure rate of one seal per 150k miles. On synthetics the failure rate was virtually zero

This mirrored my experience too

As for transmission shift forks, these can be a major wearing item (and usually due to poor Driver behaviour) - I made no replacements during my Ownership cycle when the synthetic lubricant was introduced early, in the first 2k of Ownership

 

[MolaKule]

Originally Posted By: Sunnyinhollister
Molakule,

Thanks for the info on thermal conductivity. On the Jeep boards it is taken as gospel that synthetic gear lube has LESS thermal conductance than mineral oil and the gears run hotter. Currie will actually void your warranty if you run a synthetic gear oil. Apparently there was a rash of failures and synthetic was blamed.


The question is, was there a direct correlation or cause-and-effect relation between the lubricant and the failures, and was it a particular synthetic formulation that caused this alleged failure.

I suspect a design problem (such as clearances, material quality, heat treating, etc.) and the simplest way to deflect a design issue is to blame a lubricant. I have been down this road before when developing lubricants.

 

[stchman]

Originally Posted By: Sunnyinhollister
Molakule,

Thanks for the info on thermal conductivity. On the Jeep boards it is taken as gospel that synthetic gear lube has LESS thermal conductance than mineral oil and the gears run hotter. Currie will actually void your warranty if you run a synthetic gear oil. Apparently there was a rash of failures and synthetic was blamed.


Sounds like a load of manure to me.

 

[ExMachina]

Thinking about changing the diff oil out in my car.
I'm assuming the Amsoil comparison at
http://www.lastgreatroadtrip.com/wp-content/uploads/2010/11/gear-oil-comparison.pdf
is "honest". Amsoil wins, no surprise, since its their study, or they are for real. I do admire Amsoil products.
Amsoil Severe Gear 75w-90 it will be, for lack of any better information.
Surprising how poorly Royal Purple and Lucas did !!! And, who wants a GL-5 that tests for foaming??!! Amsoil won it all there.

 

[aquariuscsm]

I searched through the gear oil UOA's here and Severe Gear always shows the lowest wear metals.

 

[ExMachina]

Originally Posted By: aquariuscsm
I searched through the gear oil UOA's here and Severe Gear always shows the lowest wear metals.

Thanks for checking that. (I didn't know bitog had a gear oil UOA, just thought there was only motor oil UOA's....)
I was about to get out today and track some down. Amsoil is solid. I admire the fact they actually have a comparison tech paper, where most other companies just say stuff like "We've got 'SinkerGlide' ", or something fluffy like that.

 

[kschachn]

Originally Posted By: Sunnyinhollister
On the Jeep boards it is taken as gospel that synthetic gear lube has LESS thermal conductance than mineral oil and the gears run hotter. Currie will actually void your warranty if you run a synthetic gear oil. Apparently there was a rash of failures and synthetic was blamed.


Stuff is always blamed for things when in reality there is no demonstrated correlation. And, a lot of things are "taken as gospel" when there is no supporting evidence either.

 

[ExMachina]

Changed the rear differential fluid with Amsoil Severe Gear 75w-90 today.
Both the fill plug and the drain plug were magnetic.
Rear drain plug had twice the iron particles as the fill plug, makes sense.
It seemed like the usual amount for 4 years and 62,000 miles, what to expect.
Below is a picture of the drain plug (I wiped half to show the thickness).