75w90 or 75w140 in diff?

[Walt]

I'm trying to decide between 75w90 or 75w140 for my differential. Right now I have the Motul 75w90 Gear 300 LS in it.

Should I switch to 140 for longevity? I don't have a temp sensor so I have no idea if the diff is overheating. I do run at fast tracks like Spa Francorchamps.

Will I notice a decrease in power if I switch to 140?

This season I didn't have problems with 75w90 but I wouldn't know if there is any excessive wear due to the heat as I have no sensor. I actually have the hole in the diff cover for a sensor but the sensor itself isn't there. I don't think there is a way to add a sensor as the wiring harnass is different I suppose?

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[Walt]

I've got the same trac-loc diff in the Ecoboost I suppose. I wanted to go with the Motul Gear competition as well but someone mentioned to me that I will see a decrease in power due to the heavier oil and slightly higher temps as well. I've "only" got about 317 HP so definitely not looking to lose any of those

 

[WildHorse]

I went 75w140 synthetic w/ 3.5 ounces of that stinky Ford shit.
Don't care if I lose .1 mpg.

 

[tosha]

I can see how thicker oil might slightly affect fuel consumption, but I don't see how it can affect HP. diff is a mechanical device and is only transferring power that is already sent to it. I doubt that higher oil viscosity can generate enough resistance to be noticed.

 

[robvas]

I can see how thicker oil might slightly affect fuel consumption, but I don't see how it can affect HP.

It's the same thing.

 

[SheepDog]

Split the difference and go with Amsoil 75w110

 

[tosha]

It's the same thing.

Are you sure?

 

[robvas]

Are you sure?

Yes. Both are measuring efficiency

Thicker oil increases drag on your engine/gears/whatever. Loses HP and fuel efficiency. Could you swim faster through water or honey?

Now, that being said the losses are minimal.

Remember the claims Royal Purple would make, where if you changed your engine oil, transmission fluid, and differential fluid to Royal Purple you'd gain 8-10hp? :lol:

This dyno graph is 20W50 vs 0W10

So you'd never see anything close to this with brands of the same viscosity or one level difference (like 5-30 vs 5-20)

Not to mention just changing the oil in your diff...

 

[tosha]

Yes. Both are measuring efficiency

Thicker oil increases drag on your engine/gears/whatever. Loses HP and fuel efficiency. Could you swim faster through water or honey?

That's understood, but I'd rather say that they correlate rather than it's the same thing. MPG is more complex, things like warmup cycles or highway cruising will have a toll with thick cold oil. You might actually see some small change in fuel consumption.

Once on track and all the fluids are in the hot zone, I highly doubt one would see any meaningful difference.

 

[robvas]

Once on track and all the fluids are in the hot zone, I highly doubt one would see any meaningful difference.

I agree

 

[Walt]

So the general consensus seems to be 75w140 for track driven cars? Or is the difference with 90 minimal and not worth the change?

 

[EFI]

Thicker oil increases drag on your engine/gears/whatever. Loses HP and fuel efficiency. Could you swim faster through water or honey?

Oil viscosity highly depends on temperature as I'm sure you know. People that put in w140 in the diff expect for it to get hot and do so many times on say a track. When the oil heats up that much, it thins down closer to the w90 weight that street driven cars see. So in reality, a street car running 75w90 at normal temps sees the same oil viscosity (and thus power) as a track car running 75w140 at high high temps.

 

[GTP]

Short answer: 75w90 if daily exclusively, 75w140 if daily/track car

So the general consensus seems to be 75w140 for track driven cars? Or is the difference with 90 minimal and not worth the change?

Oil viscosity highly depends on temperature as I'm sure you know. People that put in w140 in the diff expect for it to get hot and do so many times on say a track. When the oil heats up that much, it thins down closer to the w90 weight that street driven cars see. So in reality, a street car running 75w90 at normal temps sees the same oil viscosity (and thus power) as a track car running 75w140 at high high temps.

Long answer is as EFI said.
Without a diff cooler, your diff will run 260+ on track by the afternoon. At that temp, 140 weight behaves like 90w (or worse) anyway. You can use the attached nomograph to play What If. 12cs is a good baseline for viscosity.

Here is another chart to convert ISO to SAE viscosity:

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