Change in Gear Oil Spec for 2015 Mustangs

[SID297]

I'm not sure if many of you guys noticed that Ford has made the change from 75w140 to 75w85 gear oil for the rear diff. Ford doesn't seem to release nearly as much information about their lubrcants as some manufactures do, but I was able to quickly find the following info.


MOTORCRAFT SAE 75W-85 PREMIUM SYNTHETIC HYPOID GEAR LUBRICANT

Kinematic Viscosity @ 100°C, cSt --- 11.53

Kinematic Viscosity @ -40°C, cSt --- 48,000

Viscosity Index --- 170

Pour Point, °F (°C) --- -49 (-45)


Motorcraft SAE 75W-140 Synthetic Rear Axle Lubricant

Kinematic Viscosity @ 100°C, cSt --- 25.6

Kinematic Viscosity @ -40°C, cSt --- 135,000

Viscosity Index --- 168

Pour Point, °F (°C) --- -49 (-45)


As you can see, there's a huge difference in viscosity between the two. One thing that really annoys me is Ford reporting their viscosity at -40°C (which interestingly is also -40°F). Gear oil is essentially solid at -40°C so you don't really get much useful information from that data. They also don't include any pressure tolerance info, which is essential for a gear oil. Motorcraft tends to use Group III base stocks in their synthetic oils, so I wouldn't expect those Falex figures to be stellar. For comparison here's a some info one of Amsoil's non-premium gear oils in a viscosity close to Ford's new recommendation:

Synthetic 80W-90 Gear Lube

Falex Procedure B (ASTM D3233) (failure load, lbf.) --- 2000



So taking a look at the specs it is instantly obvious that the 75w140 is much more viscous and can handle far more pressure than the 75w85. So why did Ford move to what appears to be a less robust lubricant for its newest Pony Car? I reached out to some engineers at Ford and here are the reasons they gave for the change; "75w85 axle lubricant has less parasitic losses (better fuel economy) and runs cooler than previous lubricants. The new lubricant is less viscous in cold weather. It is used on all 2015/16 Mustangs including the Torsen and limited slip applications."

So my initial assumptions were correct, much like the switch from 5w30 motor oil to 5w20 over a decade ago this change was driven by the goal of increased fuel economy. In this case, I agree with the change. For 99% of Mustang owners I don't believe they'll ever notice the difference, the 75w85 will be fine. Modern lubricants are far more robust than those from just 20 years ago, and for the vast majority I believe the new spec oil will meet all their needs. However, let's take a look at some Premium Synthetic options from Amsoil:


Amsoil Synthetic 80W-90 Gear Lube

Kinematic Viscosity @ 100°C, cSt (ASTM D445) --- 16.8

Kinematic Viscosity @ 40°C, cSt (ASTM D445) --- 109

Viscosity Index (ASTM D2270) --- 167

Falex Procedure B (ASTM D3233) (failure load, lbf.) --- >3500


Amsoil Severe Gear® 75W-140

Kinematic Viscosity @ 100°C, cSt (ASTM D 445) --- 27.5

Kinematic Viscosity @ 40°C, cst (ASTM D 445) --- 193.2

Viscosity Index (ASTM D 2270) --- 180

Falex Procedure B (ASTM D 3233) (failure load, lbf.) --- >3500



IMO, if you are adding significant amounts of power (150+ RWHP) to your S550 Mustang or using it in extended HPDE events I would suggest switching to a gear oil like Amsoil's Severe Gear 75W-140. You may loose 1/10 of a mile per gallon in fuel economy and 1 RWHP, but the added protection you would be giving your differential would more than offset those losses.

-SID297:beer:

 

[Dcrypter]

Looks like it might be worth while to get some fluids swapped out soon.

 

[SID297]

Looks like it might be worth while to get some fluids swapped out soon.

I wouldn't worry about it unless you are adding a blower of some type or are doing HPDE. However, when people start doing gear swaps I will recommend going with 75w140 since they are already in there.

 

[Dcrypter]

I wouldn't worry about it unless you are adding a blower of some type or are doing HPDE. However, when people start doing gear swaps I will recommend going with 75w140 since they are already in there.

I've had the Roush blower on for about 2500 miles now and it's been driven hard more than a couple times heh. Seems like a good idea to give it a little more protection.

 

[SID297]

I've had the Roush blower on for about 2500 miles now and it's been driven hard more than a couple times heh. Seems like a good idea to give it a little more protection.

In that case I'd make the switch.

 

[gqneon]

This is an interesting read - I'm glad you posted this. I am adding a blower and this is such an easy swap, I only need a couple quarts and it would be a good investment.

Is there any downside to moving to a 75W-140 from 75W-85 other than fuel economy potentially? I'm just wondering if there's small ports or areas or something that would make Ford engineers who built these cars to run such a lightweight lube in the rear (maybe the Torsen Limited Slip unit needs lighter gear lube?) I don't know, and I'm not pretending to be an engineer to say which is better. The logic of better economy trumping all in production cars makes perfect sense in a marketing perspective, so I buy that part.

To me it sounds like very smart and cheap insurance until I get my blower bolted on. Can anyone think of a reason not to do so?

And if I do, do I need to add more friction modifier to the rear end after draining it? Shop manual says so, but it also says to run 75/85. And if I need to do so, can I run Amsoil additive or is it substantially different from Fords?

For reference the manual says:

"Fill the differential with 4.0 oz (0.118 L) of clean friction modifier.
Material: Motorcraft® Additive Friction Modifier (U.S.) / XL-3 (U.S.) (EST-M2C118-A)"

 

[SID297]

You likely won't notice any difference. Most premium dif oils have friction modifiers already blended in. However, if you notice any chatter you can pour in a bottle of the Ford stuff.

 

[dwaleke]

This is an interesting read - I'm glad you posted this. I am adding a blower and this is such an easy swap, I only need a couple quarts and it would be a good investment.

Is there any downside to moving to a 75W-140 from 75W-85 other than fuel economy potentially? I'm just wondering if there's small ports or areas or something that would make Ford engineers who built these cars to run such a lightweight lube in the rear (maybe the Torsen Limited Slip unit needs lighter gear lube?) I don't know, and I'm not pretending to be an engineer to say which is better. The logic of better economy trumping all in production cars makes perfect sense in a marketing perspective, so I buy that part.

To me it sounds like very smart and cheap insurance until I get my blower bolted on. Can anyone think of a reason not to do so?

And if I do, do I need to add more friction modifier to the rear end after draining it? Shop manual says so, but it also says to run 75/85. And if I need to do so, can I run Amsoil additive or is it substantially different from Fords?

For reference the manual says:

"Fill the differential with 4.0 oz (0.118 L) of clean friction modifier.
Material: Motorcraft® Additive Friction Modifier (U.S.) / XL-3 (U.S.) (EST-M2C118-A)"

I don't have the link to reference however I read Ford dropped down to a lighter fluid because 75-140 was limiting top end speed.

If you're not running faster than 150mph it's probably a non issue.

 

[gqneon]

150 is really where I start driving my car hard, especially during rush hour. Oh wait... Lol nvm.

Although the first time I saw the guy lock my car in 5th on the dyno and pull it to like 150 all I could think of was "what happens if those straps give?"

I've got two quarts of the amsoil severe gear 75W-140 and a bottle of friction modifier from Ford just in case.

I am having a ton of fun with this freaking car already lol. And I found the trunk will hold an 8-gallon hot dog compressor in the box from harbor freight, if anyone is ever so inclined to try it, on a completely unrelated side note!