The rear differential is an often neglected but essential component of the high performance drivetrain in an Aston Martin DB9.  Most people know about engine oil and transmission fluid, but what about the unsung hero of the rear wheel drive – the differential? All Gaydon era Aston Martins (DB9, Vantage, DBS, Rapide and Vanquish) have a ‘limited slip’ high performance rear differential.   It is lubricated by special gear oil (differential fluid).  For cars with automatic transmissions Aston Martin recommends it be changed every two years as part of the 2 year annual service regime (learn about that here).   I dug in and researched what the correct differential fluid.  It is different for manual or automatic transmission vehicles, so read on which one is right for your car.

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What does the Differential do?

In the simplest sense, the differentials job is to take the rotating motion of the engine and driveshaft that travels along the centerline of the car from front to back, and turn that motion 90° to the left and right to send it to the two rear wheels.

It has a special gearset within it that allows the two wheels to turn at slightly different speeds to each other (during corners), and the special ‘limited slip’ aspect is that there is a limit to how different the rotational difference is between the two wheels, ensuring some power is always being transmitted to both rear wheels even when one completely looses traction (like on ice).

If you interested in learning more about how a differential works, check out this article and animation.   Otherwise, read on to learn about the fluid it needs.

The Transaxle

First off there is some confusion normally in the online forums and groups.   Most know that Aston puts the transmission in the back of the car for perfect 50/50 weight distribution.  To package it as small as possible, Aston designed both the manual and automatic transmissions to couple directly to the differential unit.  They call this combination of transmission and differential the Transaxle assembly.

How it’s built depends on whether you have the Automatic or Manual transmission:

• If you have a Touchtronic II 6-speed automatic transmission the transaxle is made from two (2) pieces then.   The Automatic Transmission made by ZF is bolted directly to Graziano Limited Slip differential model AM803D.
• If you have a 6-speed manual or sport shift transmission – it is a one piece unit (manual transmission and limited slip differential all in one unit) made by Graziano – model AM803T.

The confusion arises that some people assume that the transaxle unit uses the same fluid in BOTH parts, the transmission and the differential.  It depends:

• For a car equipped with the Touchtronic II automatic transmission, this is NOT the case.  The Transmission uses transmission fluid (hydraulic fluid) and the Differential uses differential fluid (gear oil).  Hydraulic fluid and gear oil are most definitely not the same thing or interchangeable in any way.
• For a car equipped with the Manual or Sport Shift transmission, they do indeed share the same fluid reservoir and fluid type with the differential.  Manual transmission fluid and differential fluid are both gear oils.

This is where the confusion comes from. Automatics have separate fluids, Manuals share the one fluid.

At this point, I’m going to split the conversation to talk about Automatic Transmission and Manual Transmission Cars.

Cars with Automatic Transmissions

This portion of the discussion is for cars fitted with the Touchtronic II 6-Speed Automatic Transmission.

Aston Martin’s Recommendation

As always I start by learning what Aston Martin wants us to use.  Who better to know what’s best?  The first place I looked was in the glove box – specifically at the Fluids and Capacities page in the Owners Manual.  For the Touchtronic II automatic transmission you can see it recommends Shell ‘Transaxle’ oil 75W/90.    And it says we need about 1.6 liters of it to do a fluid change.

I also reviewed the official Aston Martin DB9 Workshop Manual Section 7.01 on the differential (read it here).   It too states Shell ‘Transaxle’ oil 75W/90.

While that spells out the brand and grade (viscosity) of the gear oil we need, it doesn’t really give us any real specs to pick the right gear oil.   Does Shell only have one gear oil that serves that purpose?  Hardly.   If we are thinking about using an alternate brand, what standards must it meet?

Shell Spirax S6 AXME Gear Oil

To find out I purchased a bottle of Aston Martin recommended Shell Transaxle gear oil from a main dealer.   As you can see from this photo of the label on the bottle, it is Shell S6 AXME 75W-90 gear oil.   It even has an Aston Martin logo on the bottle allowing them to jack the price through the roof.

Most importantly, this reveals the industry standard specs we would be looking for:

• 75W-90 Viscosity
• API Standard GL-5 and MT1

With that in mind, I then set out to find a way to purchase Shell Spirax S6 AXME online.  And mostly struck out.  In the USA at least, this is not a mainstream retail product.  My only option if I wanted this specific fluid was to purchase from an Aston Martin dealer or an Aston Martin parts supply company like ScuderiaCarParts.com.  Perhaps where you live this will be easier to find.

Other Brands

Lots of companies make a quality 75W-90 gear oil that meets the API GL-5 and MT-1 standards.

My search discovered Castrol Syntrax 75W-90 Gear Oil designed for use with limited slip differentials.  It meets or exceeds API GL-5 and MT-1 standards and states “suitable for limited slip differential axles” (find the Product Data Sheet here).  This is the differential fluid I have been using for 5+ years.

Motul is a premium brand often mentioned in the forums and they make Gear 300 LS 75W-90 gear oil.  Pay close attention to the LS in the part number, this indicates the version made for LSDs.  They make a version without the LS, so watch out.  Full Synthetic, GL5, 75W90.  Appears to check all the boxes.  I’ve not used the Motul in my own car.

Limited Slip Differential Additive

As I have mentioned above, limited Slip differentials require more than standard gear oil.  They have a clutch pack literally submerged in the fluid, and to get the proper level of ‘slip’ and ‘grip’ they need a special additive.   The Shell Spirax and Castrol Syntrax have this already in the fluid.   If you use a regular gear oil (like Motul) you can augment it to be limited slip compatible by adding a special additive.

The recommended additive is Kendal Limited Slip Differential Additive.  It is sold in small 4 oz. bottles with a recommended mixture ratio of 2 oz. per liter of gear oil.  So one bottle is enough to mix with two liters of gear oil.

I spoke with an Aston Martin Master Technician about what he recommended with the additive.  His advice was to use the Shell Spirax or Castrol Syntrax, and no additive would normally be needed.  But, if you were feeling differential ‘judder’ when the car was cold and you were turning sharply coming out of a driveway onto the street, then the additive would cure this.   He also commented that in his experience a well maintained differential that has had the correct fluid changed every 2 years as specified generally will not need the additive.   An ill maintained differential is more likely to have the gear chatter.   Morale of the story – maintain your differential with the right fluid on the right schedule.

Cars with Manual Transmissions

If your car is equipped with the 6-speed manual (you lucky duck) or sportshift transmission remember that I noted above that your differential and transmission share the same fluid reservoir.

What does Aston Martin Recommend?

Opening the glove box owner’s manual in for my 2005 DB9 Coupe to the Fluids and Capacities page reveals this:

Shell Transaxle Oil 75W90, essentially the same as noted for the differential in the automatic. Keep reading – Aston changed their minds on this.

I also checked the Official Aston Martin Workshop Manual Section 7.03 on the Manual Transmission.

Same recommendation, Shell ‘Transaxle Oil’ 75W90. And it points out that a total of 4.5 liters would be needed total.

At this point it seems clear, but then a problem developed….

Field Service Bulletin SB 0236

Aston Martin started to hear complaints nearly immediately about difficulty shifting from first to second gear when the car and fluids were cold. The shifting would baulk making it difficult to engage 2nd gear. After 10 minutes of driving and the fluids warming up normal shifting would resume.

This prompted Aston to release field service bulletin SB0236 to the dealerships warning them of the issue and recommending a revised gear selector lever and a different transaxle gear oil as the solution.   You can read more about SB0236 here on Bernand Embden’s page that shows all the details of the lever change as well.

The new fluid is Castrol BOT 270a (Aston Martin part number 6G33-75106-AA/S). This is a bespoke fluid to Aston.  What are the spec’d of this fluid? Here are pictures of the front and back of the bottle:

It is a lighter weight gear oil (thinner) listed as a straight 70W vs. the nulti-weight 75W90 of the original spec Shell Spirax. This was a compromise to improve the shifting when cold. The downside of the bargain might be that if you track your car considerably (or drive long stretches at very high speeds) where the gear oil is getting very hot, this lower weight oil may not be as good for your transaxle as the original heavier weight gear oils. Most Aston’s never see the any substantial track use, so this was likely an acceptable tradeoff for most owners.

It is described as a full synthetic API GL4  gear oil.  The original Shell gear oil was a GL5 and MT-1 specification, and you can learn a little more about that here on Wikipedia.  GL4 is rated as less heavy duty than GL5.

There is also an interesting opinion here on 6SpeedOnline by user Irish07 (Chris Edgett, an ex Aston Master Technician in Canada that now works as the Technical Head of Velocity AP).  Check out the post here.

Back to our discussion of the differential fluid. From what I can tell Castrol BOT 270a is not specified as for use with a Limited Slip Differential.   I also spoke with Mike from Bamford Rose.   Bamford Rose is an independent Aston Martin specialist in the UK.  Mike’s recommendation was straightforward, always use the Castrol BOT 270A along with the Kendall LSD Additive.

Costs

I tracked down the costs for the various fluids for either scenario

• Gear Oil for the Differential in cars with Automatic Transmissions (buy 2 one liter bottles)
  • Shell Spirax S6 AXME 75W-90
    • I called my nearest Aston Martin dealer and inquired.  $29 USD per liter (plus tax).  The catch was they were NOT willing to ship it.
    • ScuderiaCarparts.com sells a 12 Pack of this for $281 USD.  That’s about $24 USD per bottle.  Might make sense if you plan on having your car for more that a decade or can share with your other DB9 owner mates.
    • AstonParts247.com in the UK has it available online for £16.00 (excl. VAT) per bottle.
  • Castrol Syntrax 75W-90 for Limited Slip Differentials
    • Available online from Amazon.com for as little as $15 USD per one liter bottle (including free shipping) when purchased in a six pack.
    • Note: They make a version of Castrol Syntrax not for use with LSD’s, so make sure you get the right stuff (say Limited Slip right on the front label).
  • Motul Gear 300 LS 75W-90 for Limited Slip Differentials
    • Available on Amazon.com for about $26 USD per one liter bottle (including free shipping).
  • As you can see then for as little as $30 USD you’ll have what you need to do a fluid change.
• Gear Oil for the Differential in cars with Manual Transmissions (buy 5 one liter bottles).
  • This is difficult to track down anywhere but at an official Aston Martin dealership since it is a bespoke fluid, but a few other places sell it.
  • Castrol BOT 270a  (Aston Martin p/n 6G33-75106-AA/S)
    • I called my nearest Aston Martin dealer and inquired.  $77 USD per liter (plus tax). Yowza!  That’s $385 USD for the five liters.  They said they were NOT able to ship it.
    • ScuderiaCarParts.com offers it online for $36 USD per liter (plus tax and ship)
    • Redpants.lol offers it online for $39 USD per liter (plus tax and ship.
    • In the UK you can purchase it online from AstonMartinBits.com for about £20.80.
  • Kendall Limited Slip Differential Additive
    • They recommend 2 ounces of additive per liter of gear oil.
    • Two 4 ounce bottles should be enough to go with the 4.5 liters of gear oil used in a manual transmission fluid change
    • Available online here from Amazon.com with free shipping for about $15 USD per bottle.
  • If you buy it all online you are looking at about $225 USD for all the fluids.

Aston1936 Recommendation

It depends on your car and availability of fluids:

• If you have a manual transmission or sport shift your best option is the Castrol BOT 270a combined with the Kendall Limited Slip Additive.
• If you are like me and have a Touchtronic II automatic transmission:
  • If you can get the Shell Spirax S6 AXME locally where you live for a reasonable price (Maybe in the UK or Canada), this might be a good option.  No additive is normally required.
  • If sourcing the Shell gear oil is difficult and expensive, my recommendation is to go with the Castrol Syntrax 75W-90 for Limited Slip Differentials.  No additive is normally required.
    • This is the fluid I use.
  • Use the Kendall Limited Slip Differential additive if you chose a gear oil not specifically designed for a limited slip differential  OR if you are feeling gear chatter in a hard turn as I noted in the discussions above.
    

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Video

If you’d like to see me try and explain this a bit better, and have a quick discussion with Mike from Bamford Rose about differential fluids, check out this short video..