Vehicle is a 2002 XKR with 90k miles. Diff was changed by previous owner 7 years ago. When I pulled a sample of the oil it was not in great shape (no metal but the viscosity was breaking down)….so needed a replacement.

I found a LOT of discussion about what to choose, but I wanted to stick to the manufacturers design (thicker is not always better). Here is what I found. Capturing my notes if nothing else. If you spot a mistake or have some improvements please let me know!

TL;DR:

75W140

Synthetic

Zinc (ZDF) additive

Royal Purple/Lucas


IN DETAIL:

First up - the weight....

The original Jaguar Spec is Thermally Stable Super 90 - no longer available...but oil is oil so let’s find the specs. We want the viscosities and the additives used to make up the properties of this particular oil...

Spec sheet here: https://www.jaguarforums.com/forum/a...per-90-tds.pdf It is dated march 2004 and has a table of viscosities on page 4. The table itself says the stuff is SAE-90…but we want the details.

The viscosity at temps are:

220.0 cSt @ 40*C

19.5 cSt @100*C

Loosely translated...that is the XXW-XX number on the SAE oil. The W stands for Winter…

Cross referencing 220 cSt and 19.5 cSt into SAE... I get 75W-90….but that does not account for oils breaking down a tad. We can of course go to a higher viscosity like 75W-140 but you get higher friction and less flow. Note that both 90 and 140 will be in the “hydrodynamic” range meaning the oil is always between gears at speed. I want to get out of metal-metal contact quickly and keep friction low….there is a “sweet spot” for oil thickness known as the Stribeck Curve. https://en.wikipedia.org/wiki/Stribeck_curve

I looked at RP, Mobil 1, Lucas, and Castrol, but RP had the best fit. Using this chart from Royal Purple for their oil range: https://www.royalpurple.com/wp-conte...axgear_pds.pdf

The original specs (220->19.5) do not seem to match up to SAE 90?! 75W-140 had the best fit to the original specs for the RP brand. Accounting that oil breaks down after longer use and my gears are probably worn a bit…. I’m going to push to the 140. It is 187@ 40C and 27.5 @100c.


Ok – on to the additives. Properties in the sheet are:

1. Anti-Pitting

2. Shear Stability

3. Fretting Performance

#1. Anti-pitting in hypoid gear oil is accomplished by selecting a particular oil…but can also be done by adding a low-friction components, dispersants, . Research done in 2019 by university of Sweden tested all these by running them at speed in a gear meshing situation. Our oil notes it is “API Service Classification GL-5” for additives and that type got tested. GL5 is in Lucas oil, RP and Mobil 1.

Looking at the test results, the PAO (synthetic) oil fared better in anti-pitting across the whole speed range. SAE 90 oil fared better than SAE 80 oil...but only marginally.

Viscosity modifiers had no impact. The oil tested without modifier had the lowest friction.

• Friction modifiers ended up having no impact.
• Anti-wear additives (zinc dithiophosphate) did reduce pitting.

…the research tests here: https://www.tandfonline.com/doi/pdf/...eedAccess=true

#2. Shear stability is all about how long it holds viscosity over repeated use and at temperatures. Usually a high Viscosity Index (VI) is what you are looking for. It is accomplished with certain polymer additives. Our spec sheet shows a VI of 101…pretty high! https://en.wikipedia.org/wiki/Viscosity_index. I poked around to find ratings and you get a ton of variance and found this: Amsoil Synthetic Lubricants | Gear Oil comparison test

Lucas and Royal Purple seem to have pretty good additives. Here’s the specs:

https://www.royalpurple.com/wp-conte...axgear_pds.pdf

https://lucasoil.com/pdf/TDS_Lucas-S...SAE-75W-90.pdf

I noted that the 75-90 and 75-180 had a nearly identical VI.



#3 Fretting performance is a measure of wear when the gears are in contact as a corrosion process starts up. The main culprit is fine powder debris on the mating surface from internal corrosion.

Anti-wear ZDDP + dispersant had the best results. Dispersants are fun things like Boron, Magnesium, barium…the total dispersant PPM is what I went looking for. Again I found Lucas/royal purple coming out on top…bu that will change.


…the tests are here: https://link.springer.com/article/10...49-020-01379-6

Adding all that up:

• The weight appears to be more 140 than 90
• A modern high-end oil like RP, Lucas, or Mobil 1 will have additives on par with Shell in the late 90's early 2000's.
• Friction modifiers did not help much
• A good zinc additive will add to longevity

I have some options between RP, Lucas and Mobil 1. From those I selected RP, and 75W140. Your results may vary of course!

Hope that helps in your oil selection...

 

I appreciate all the effort you put forward to compile this and share it....thank you!

I have used Red Line products for many years, if you have a chance would you browse their dif oils and post your impression as to whether or not one is appropriate for our x100 dif?


https://www.redlineoil.com/gear-oil-for-differentials
'

wj

 

Full synthetic Mobil 1 75w-140 is working very well in my ‘02 XKR.

Changing it was one of my first maintenance tasks when I bought the car 3 + years ago. The rear end immediately became quieter, and 50,000 miles later the level has not changed, nor has the appearance of the fluid.

PS. my sympathies for the convertible owners re changing the diff fluid. The process in my coupe, while still having a very tight access , was not that difficult in comparison to what I’m learning the convertibles owners go thru.


Z

 

Most Welcome!

I use redline in my F355 since it matches the original shell spec more closely....sadly i do not have it in my local area right now for the XKR, thus why I skipped it :-(

Here's what I found: https://www.redlineoil.com/technical-info

75W90 High Performance Gear Oil: Viscosity: 16 cSt @ 100°C; 112 cSt @ 40°C - thinner than what we would need to run. It would work of course but is less of a fit.
75W140 High Performance Gear Oil: Viscosity: 26.9 cSt @ 100°C; 162 cSt @ 40°C - note that the low-temp is thinner than spec by about 20%.

...both are lower than the RP/Lucas PPM on the anti pitting and shear stability results, but higher than Mobil1.

They certainly work, I'd suggest the 75W-140 is the closest fit to the original Shell spec. Note that there are "NS" versions of both...those are for limited slip diffs (I run them in the F355) but you should avoid them for the Jag.

...I hope that helps!

 

many manufacturers use open diffs with fluid containing limited slip additive.

either way this differential is junk, just make sure it doesn’t run out of oil

 

Wow, awesome research Clint. I have the 2000 XKR convertible, 50K miles, have not checked the gear box as of yet for any reason, but this will help when I put my cat on the jacks for some preventative maintenance and will check all fluids.
thanks
Dave
XKR conv. 2000.

 

Good info, and wouldn't be directly so concerned with the 140 ones. The Shell one was a long life version, and iirc smelled highly sulphurish compared to others, but its more then a decade ago that I changed that one.

I am actually using Os Giken 80w-250, as that was the recommended one for my LSD:

SPECIFICATIONS

• Type: Full Synthetic
• Use: For automotive gear
• SAE: 80W-250
• API: GL-5
• Flash point: 208C
• Kinetic viscosity: 320.5 (40C), 43.7 (100C)

Not that I would recommend this one for the open diffs, but more to show what one could run even.

 

All good Dave! One of those "while it's on the stands" things: if you can borrow/buy a grease gun the drive shafts have 4 lube points you can get done easily (if you have the grease!). I forgot and had to pick up some lithium grease for the gun....