Gear oil 80W-90 versus 75W-90

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Before SAE split the gear oil grades, 75w90's weren't as popular.... and some 80w90's were as thick as current 110 grades

90 grade long ago 13.5-24
90 grade today 13.5-18.5 with 110 now covering 18.5-24cst
Spot on and what most people don't know.

When Ford rebuilt the rear axle, under warranty, in my Ranger they refilled it with 75w-140. This got me to wondering why Ford made the change which led me to their TSB mentioning that all rear axles to be filled with 75w-140. This led to another search as to why the change? What I came up with was the old/new spec's Greasymechtech mentions in the new SAE directives. I believe Amsoil was the first to come up with 75w-110 (and others may have followed) to cover the old 75w-90 spec's.

I decided to follow Ford's direction and replaced all my vehicles rear axle oil's (they are old and fall under the old spec's) to 75w-140 with a caviet. I chose to go with synthetic gear oil because of the vast difference in [email protected]

M1 and SuperTech synthetic's are very close in spec but I will show SuperTech since that is what I chose:

Synthetic = [email protected] 171.8 VI 200
Conventional = [email protected] 374.7 VI 100

So you see there is a huge spread between the [email protected] and why I chose synthetic.

75w-110 is an excellent replacement choice for older vehicles that spec'd 75w-90 but 75w-140 is easier to find.
 
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I converted from F to C to be able to easily compare to what you were saying. In F, my rear axle temps in 95°F ambient temperatures run between 180°F and 225°F.

I measure the oil temperature directly because I have sensors mounted inside the front and rear axle covers. I run 75W-140 in the rear axle and 75W-110 in the front axle.

My point is that temperatures that are more than 20° C difference between ambient and the actual oil temperature are not a problem. Moreover, I would be skeptical if they were not, because I have never seen an axle under load operate within a 20° delta from ambient.

Last, but not least, I would use 75W-110 instead of 75W-90 or 80W-90 because 75W-110 is the upper viscosity that 75W-90 used to be before they updated the J306 tables a while back.
Here is some info from Banks Engineering regarding diffs temps. It was from a truck towing a heavy load up a hill. They were marketing their new diff covers. 200 F was not uncommon for a stock diff temp when under load.

0D3305A3-C003-40D0-804B-23F7FD855118.jpg
 

SoNic67

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That's probably an extreme example, marketing purpose.
So again... with the changes made to the 75W-90 standard (lower viscosity) and those temps, why would anyone won't use the 75W-110 in place of 80W-90?
 
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Spot on and what most people don't know.

When Ford rebuilt the rear axle, under warranty, in my Ranger they refilled it with 75w-140. This got me to wondering why Ford made the change which led me to their TSB mentioning that all rear axles to be filled with 75w-140. This led to another search as to why the change? What I came up with was the old/new spec's Greasymechtech mentions in the new SAE directives. I believe Amsoil was the first to come up with 75w-110 (and others may have followed) to cover the old 75w-90 spec's.

I decided to follow Ford's direction and replaced all my vehicles rear axle oil's (they are old and fall under the old spec's) to 75w-140 with a caviet. I chose to go with synthetic gear oil because of the vast difference in [email protected]

M1 and SuperTech synthetic's are very close in spec but I will show SuperTech since that is what I chose:

Synthetic = [email protected] 171.8 VI 200
Conventional = [email protected] 374.7 VI 100

So you see there is a huge spread between the [email protected] and why I chose synthetic.

75w-110 is an excellent replacement choice for older vehicles that spec'd 75w-90 but 75w-140 is easier to find.
Could I see the TSB?
 
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That's probably an extreme example, marketing purpose.
So again... with the changes made to the 75W-90 standard (lower viscosity) and those temps, why would anyone won't use the 75W-110 in place of 80W-90?
Chevrolet is quite aware of what the viscosity is of a modern synthetic 75W90. Ford seems to like 75w140. Obviously the two manufacturers do not agree. As for the RAV 4 , Kia Soul, and Hyundai Sonata, any fluid would work as there is hardly any load happening and the temp shown was less than 111 F. Pick your viscosity and go for it. If that answer is the thicker the better, so be it. I have over 200,000 miles on a diff using synthetic 75W90. Thanks for your charts.
 

SoNic67

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Yeah, I wasn't asking what to use. I was just sharing in a table various gear oil viscosity, obviously everybody can choose what they want.
I even stated that in the first post.
 
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That's probably an extreme example, marketing purpose.
So again... with the changes made to the 75W-90 standard (lower viscosity) and those temps, why would anyone won't use the 75W-110 in place of 80W-90?
Not extreme; quite common. On the freeway today in 65°F ambient temperatures, my rear axle was running at 175°F. My 2010 F-150 FX4 and 2015 F-250 Powerstroke had similar temperatures.

Not sure why you think these temperatures are not normal.
 
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I think people are way over thinking this. I don't think I've ever seen any study or data suggesting that 80W90 vs 75W90 will ever make a difference over the life of the vehicle, even in extreme -40C operating conditions. Yes, in principle there are viscosity and temperature differences between the brands that can be measured, but I really doubt they would vary enough to make a difference in a consumer application.

Maybe in heavy trucking, or industrial applications where the gearing is expected to run close to 24/7 for years, then you might have measurable wear differences that matter.
 

SoNic67

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Not extreme; quite common. On the freeway today in 65°F ambient temperatures, my rear axle was running at 175°F. My 2010 F-150 FX4 and 2015 F-250 Powerstroke had similar temperatures.
So what's your point? That because you have hot usage yo never run colder? Like at start-up, in winter... Do you argue that thinner oil at that temperature is fine, because it can be hotter? IMO that is more argument for 75W-110 as replacement for 80W-90. The 90 of synthetic is not equal with the 90 from mineral.

I think people are way over thinking this. I don't think I've ever seen any study or data suggesting that 80W90 vs 75W90 will ever make a difference over the life of the vehicle.
You don't think it doesn't matter. Others might think it does. I would say that include the car manufacturer that actually specifies a certain fluid for the application.
If they said 80W-90, they might have something in mind, is not like they didn't know that 75W-90 exists. Also they didn't say "use whatever you feel like".

Like I said, I posted the table so everyone can make informed decisions, not to argue a way or another.

I also said that for me it made more sense to replace 80W-90 with 75W-110 because of similar viscosity at 40C and better viscosity at 100C (close to what initial 80W-90 meant).
 
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So what's your point? That because you have hot usage yo never run colder? Like at start-up, in winter... Do you argue that thinner oil at that temperature is fine, because it can be hotter? IMO that is more argument for 75W-110 as replacement for 80W-90. The 90 of synthetic is not equal with the 90 from mineral.
My point is they DO NOT run colder--oh it might until the axle is warmed up, but you are not going to see any axle running down the highway for any length of time at 110°F (as your heat reading shows). I have also never seen an axle that only runs a 20° delta between ambient temperatures and the oil temperature.

I am not arguing the viscosity--I run 75W-140 in the rear and 75W-110 in the front and am one of the few who does because I know that 75W-110 is the upper viscosity that xx-90 used to be before the J306 tables were updated splitting the viscosity between 75W-90 and 75W-110. Of course higher viscosity provides more protection in higher temperatures and of course synthetic oil can handle higher (and lower) temperatures than conventional oil.

Which leads me to ask--what is your point?
 

SoNic67

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My point? I said it in the first post.
I see a lot of people asking if their specification 80W-90 can be substituted with something else. Invariably the chorus here says "use 75W-90 is better because muah, synthetic".
So I made a table in which it is visible that 75W-90 is not always identical or better with the specced 80W-90.

Also, the wear of gears, while pumpkin is warming up, or in cold ambient weather, might be higher than the one at operating temperatures, when dilation makes things tighter.
That certainly is true for engines in warm-up condition.
 
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who here has actually worn out a differential? i have broken a few but no fluid in the world was gonna protect a BOP 8.2 from a 6000 rpm clutch drop. i mean don't most go at least 200k on the factory fill?
 
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