Ford switch 80w-90 to 75w-140 synthetic. Why?

[j_mac]

Hello,
I've owned 20-some V-8 Mustangs over the years. They always took 80w-90 gear lube in the rear ends. All of my cars had the same 8.8 inch rear end.
About 2001 Ford switched over to 75w-140 Synthetic in these rear ends.
Does anyone know how and why they did this? Seems like to me it would only serve to lower fuel economy using a lube that is 140 weight at operating temp.
Did they make other changes inside the rear ends on these cars to compensate? Was it simply to increase durability?
Maybe I don't understand how gear lube works in a rear end or something.
Any and all comments would be appreciated!

 

[Bluestream]

For Durability. Check the search lots of other posts on this one.

 

[Colt45ws]

SAE split the 90 grade range in 2001. Bottom half of the 'old' 90 stayed as 90, but the top half of the range became 110. 110 is still rare and hard to find so they probably went straight to 140.
140 split the same way; top half is now called 190 grade.

 

[j_mac]

Colt 45,
Can you re-describe that to me in layman's terms. I don't understand.
What do you mean the SAE "split" the grades?

 

[Colt45ws]

I thought I was. Grades are just arbitrary numbers assigned to make it easy to select what oils you need. For example, the engine oil and gear oil grades overlap quite a bit even though engine oil tops out at 60 and gear oil starts at 80. If you were to classify most 5W20, 5W30, 10W30 engine oils under the gear oil viscosity classification, they would be an 80. The reason for this huge discrepancy is so people dont need a 10W40 gear oil and buy 10W40 motor oil instead. The additive difference is huge. So it would call for a 80W90 instead.
Viscosity is actually measured today with centistrokes (cSt)
Water is 1cSt.
Before 2001 or so these are the grade ranges of 90 and 140
90: 13.5cSt to
140: 24cSt to
Now:
90: 13.5cSt to 110: 18.5cSt to span>

140: 24cSt to 190: 32.5cSt to span>
You can see how the top half of these viscosity specifications have become their own grades. A 110 grade like I run would be classified as a 90 under the 'old' system. I suspect Ford felt the new 90 is too thin, considering the expected use of a Mustang. For instance, my 2004 Crown Victoria LX with the same 8.8" axle specifies 80W90. But, its not a Police package. They dont expect blue hairs to be doing burn outs and ripping down the drag strip; or even flooring it on to the highway. Police Interceptors, however, specify 75W140 as well. The reasoning behind a synthetic instead of dino is dino 140s tend to be 85W140, which might not have good enough cold tempeature performance for this axle. They probably would specify a 110, but like I said, its rare. Its kinda a chicken and egg thing. No one specifies it because its not around. No oil companies are making it because its not specified in any applications.

 

[Falcon_LS]

Motorcraft High Performance Rear Axle Lubricant 75W-140 has a viscosity of 25.6 cSt at 100°C. Couldn't find a data sheet for the Premium 80W-90 to compare though.

 

[Jim Allen]

Colton- That was an outstanding presentation of useful info.

 

[unDummy]

http://www.amsoil.com/storefront/svt.aspx
http://www.joegibbsdriven.com/products/gearoils/sgo.html
http://www.klotzlube.com/techsheet.asp?ID=32
http://www.q8oils.co.nz/Images/Data/Automotive/Q8 T66 75W-110.pdf

Its pretty simple. When you add weight and power, and make the rear-end as cheaply as possible, extend the change interval indefinitely, you need to protect it somehow. A thicker synthetic is the way to go with many 'ends and gearboxes.

Obviously, making something last was more important than the loss of MPG. Makes you wonder!!!

 

[j_mac]

WOW Colt!

Are you telling me that the SAE has changed the way they apply the grade weight on the bottle? In about 2001?

So, a bottle of 80w-90 bought in 1998 might have different flow characteristics at certain temperatures than a bottle of 80w-90 bought in 2010?

 

[Colt45ws]

Falcon: Its at the bottom end of the scale. I suspected as much. It will probably shear in service to a 110. Which is probably just fine.
Jim: Thanks.

j_mac: Yes, its possible. If the 1998 version was made on the thick side, say 20cSt, it could very well be different than a 80W90 today.

 

[Steve S]

Originally Posted By: Jim Allen
Colton- That was an outstanding presentation of useful info.
+1 thankyou!!!

 

[unDummy]

SAE J306 covers gear oil spec's.

http://www.lubrizol.com/DrivelineAdditives/AutomotiveGearOil/J306Specifications.html

 

[j_mac]

Colt,
I'm understanding now. But now I'm wondering if the old 80W-90 Motorcraft I always used in the older cars is the same product you buy today at the Ford dealer.
I mean, if I put a new bottle from the Ford dealer (80W-90) in my older mustang is it gonna be a similar viscosity to the stuff I used all through the 1990's?
Could it be that the old 90W oil was near the top of the range (24) and under the new SAE spec ford decided to go with a 140 that is at the bottom of the range (24) in the new specs? In effect you would have the old 90 and the new 140 with the same viscosity at operating temp.

I'm I on the right page?

Thank you for the VERY valuable info that I had NO IDEA about 2 days ago.

 

[Colt45ws]

Its possible, and what Im getting at, but I dont know if its correct. A old bottle of Motorcraft 80W90 would have to be hunted down and tested to see what its viscosity is in order to find out. Or alternatively, a old data sheet would suffice.

 

[Shuttler]

Originally Posted By: Colt45ws
Viscosity is actually measured today with centistrokes (cSt)
Water is 1cSt.
it's Stokes, like in 'Different Stokes for different folks' - after mister Stokes.
This error does not diminish high informational value of your post.

 

[Saleen0679]

Ford issued several TSBs regarding rear end noise on certain vehicles. The fix was to replace the clutch pack and change the fluid from 75w-90 to 75w-140. I believe the latest TSB was 04-24-20. There was a previous TSB 04-17-5.

 

[Colt45ws]

Oh, okay, I had an extra 'r'. Ill try to remember that in the future. Thanks.

 

[j_mac]

...little "r" or not Colt, I'm learning alot here.

Please, give me your thoughts.

 

[dailydriver]

There are some synthetic 75W-140s which are almost as thin at operating temps as some dino 80W-90s.

 

[j_mac]

This makes for a lot of new questions about what lube to put in my old 5 Liters and my '99.

So gear lube grades and viscosities may be different now than what we've grown up on. Am I understanding this right?

For instance; Say a bottle of Valvoline 80W-90 from 1995 could be reclassified as a 110 or a 140 after the SAE changed the parameters by which gear lube is measured even though it is the exact same oil it always was.

Right?