75W/90 Gear Oil is the same as 5W20 oil. NOT

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I can't tell you how many times I've read/been told 75W90 gear lube is the same viscosity as 5W-20 motor oil. I'd like to put that to bed and move on. (pending correction by those that know more than I). However, Mr. Widman, who's data is used below, is mighty good himself. Let's look: [img]https://www.widman.biz/English/Tables/ISO-SAE.html[/img] As you can see the bottom of 75W gear oil viscosity starts at 10W, and it could be as thick as 20W and beyond (lets call that 25W for now). At no time is it ever in the 0W to 5W range. None. Obviously And the potential range is quite large as well, so it's clearly not safe to say "any 75W90" is.... OK, how big a deal is this? Here are some reportedly average numbers: On just a mildly cool day you're seeing about 230 cSt for 75W90. [img]https://www.widman.biz/English/Tables/gr-dif.html[/img] On that same day we're seeing 5W30 (I don't have 5W20, but it's likely lower) at roughly 170 cSt. [img]https://www.widman.biz/English/Tables/gr-motores.html[/img] So not the same right? Is that significant? I'd need Mola to tell us, for I certainly don't know. I can see it sure seems like 10W40 is the closest cold match to 75W90. (again the viscosity range of 75W90 is so wide the data could also be skewed as a result). Now this most certainly doesn't address sheer strength or additive packages of a good gear oil, but that just not a viscosity discussion. Not do gear oils usually see the kind of temps an engine carry's so heat isn't likely to be as significant an issue as correct viscosity. At the very least I hope we can agree 75W90 is not generally the same viscosity as 5W20. I do want to leave open the probability that there are some gear oils, perhaps synthetic oils, that could meet or approach that of 5W/20. Redlines 75W80 seems to exceed the 5W/30 threshold, as does their MT fluid. I'm sure there are others too. Note however they are GL-4 oils, not GL-5. Nor does that suggest they are getting to 5w/20 weights. As a tangent, I have found a GL-5 rated oil that gets close to 5W/30: Redline 75W85 is slightly above 5W, but below 10W in cold, and beats both in both sheer and 100C temp. Close enough I guess we could say that at least one 75W85 Gl-5 rated oil is "close enough" to 5W-30. 5W-20? I don't know. If I can get Mr. Widmans graphing tools working I'll post the graph. Respects to all, -d
 
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Originally Posted by motor_oil_madman
a 75w90 is about the equivalent of a 50 weight, but gear oil doesn't have the same additives engine oil does.
uh no.. no its not.
 

MolaKule

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Quote
Even though an SAE 90 weight gear oil and a SAE 50 weight engine oil have a similar viscosity of 18.75 cSt, it is their additive packages that differ greatly for the most part. In some situations, the transmission manufacturer of heavy equipment may specify an SAE 50 weight engine oil, but this is the exception for daily driver automotive specifications. Most gear oils also contain thickeners and Viscosity Index Improvers.
Gear Tribology and Lubrication I
Quote
Basic Gear Lubricant Requirements The primary functions of a gear lubricant are to reduce friction and provide cooling. Most of the friction reduction, and certainly the cooling, is provided by the base oils formulated into the gear lube. Secondary functions, such as protection and enhancement of the gear lube, are provided by the additives. Elemental chemicals and synthesized organics provide rust protection, oxidation inhibition, metal deactivation, and increase the Viscosity Index for wide temperature swings. Additional additives for foam suppression, water demulsifiability, and seal compatibility are also important. The most pronounced additives are the anti-wear protection (AW) additives and the anti-galling/anti-scuffing protection provided by the extreme-pressure (EP) additives. Some fluids use Viscosity Index Improvers (VIIs) to broaden the temperature range of operation if the base fluid does not have its own inherent VI. In limited-slip differentials, with clutches based on inertial mechanisms, additional Friction Modification is provided by natural or synthetic esters generally called fats.
Gear Tribology and Lubrication II
 
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A 90 grade gear oil is 13.5-18.0 cSt which runs from a light 40 grade engine oil to a light 50 grade. The biggest issues with gear oils are foaming inhibition and extreme pressure. This is one area where conventional oils have the advantage over group IV PAO because the higher pressure-viscosity coefficient of group I and II oils allows them to better withstand the shock loading seen in rear ends and gears. This is especially true in drag racing. NHRA Top Fuel guys, that are slamming 10,000+ hp to the rear ends, are using an SAE 250 grade group I/II blended gear oil.
 

dafish

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Guys, did we drift off thread? Or am I wrong (again). This is about 75W90. Not the same weight particularly at transmission temperatures, as a 5w/20 and certainly nothing like 90W gear oil either. I don't doubt 90W (J306) is in the 50W range ((J300) range, (heck look at the linked table), but that's not the concern either. Mola: Thanks for the links Not sure who you intended them for, but I'm going to take advantage of them! smile
 

MolaKule

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Originally Posted by dafish
Guys, did we drift off thread? Or am I wrong (again). This is about 75W90. Not the same weight particularly at transmission temperatures, as a 5w/20 and certainly nothing like 90W gear oil either. I don't doubt 90W (J306) is in the 50W range ((J300) range, (heck look at the linked table), but that's not the concern either. Mola: Thanks for the links Not sure who you intended them for, but I'm going to take advantage of them! smile
What IS your concern or point? The links and quotes are for educational purposes.
 
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I wouldn't take the opinions of your buddies too seriously. You are comparing apples and oranges.
 
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I don't understand the purpose of this thread. As Mola referenced, a GL-4 or GL-5 gear oil is additised much differently than engine oil. Substituting either for the other would cause imminent desctruction. So this appears to be a moot argument. Conversely, I do get the viscosity curiosity as since we were kids we've been told that gear oil is very 'thick" and likely not even close to SAE 50 and that is surely not true with modern light-duty multigrade hypiod lubes which resemble a 15W40 but show a much lower pour-point to below -40C Funny that I have ST 75W90 in my Subaru diffs and my silent self noted that it "pumped like water" when I put it in this past late Spring but continues to perform admirably compared to the absolutely terrible Mobilube HD A 85W90 that was in there:)[b][/b]
 

dafish

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Point: This is a reference post. Most readers here know well the add pacs between a gear oil and a MO make a large difference. That's far from true in other forums and discussions. With respect to all, lots of those same folks won't read and think too much. They want it short and simple. I grew tired of it and wanted a shorter summary reference post I could simply link too. BITOG, in that If I'd misspoken it would quickly be corrected, was the logical site. Thanks all!
 
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Originally Posted by dafish
Point: This is a reference post. Most readers here know well the add pacs between a gear oil and a MO make a large difference. That's far from true in other forums and discussions. With respect to all, lots of those same folks won't read and think too much. They want it short and simple. I grew tired of it and wanted a shorter summary reference post I could simply link too. BITOG, in that If I'd misspoken it would quickly be corrected, was the logical site. Thanks all!
I have no idea what that means. Can you translate? Do you mean you wanted to post your wisdom here so you could link to it on other sites? Kind of like your oil filter wisdom in that other thread?
 

MolaKule

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Originally Posted by dafish
...So not the same right? Is that significant?...I can see it sure seems like 10W40 is the closest cold match to 75W90. (again the viscosity range of 75W90 is so wide the data could also be skewed as a result)...
https://www.bobistheoilguy.com/viscosity-charts/ As others have alluded to, what, if any, is the purpose of comparing engine oils to gear lubes except maybe as an exercise in comparing viscosities of differing lubricant applications across temperatures? Fact #1: SAE gear lube grades span a rather wide range of viscosities. Fact #2: There is little comparison between engine oil Di additive packages and gear oil DI additive packages.
 
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Originally Posted by MolaKule
what, if any, is the purpose of comparing engine oils to gear lubes
A practical thread on this very subject came up a while back without any resolution. A person said they added about 3 0z of Redline synthetic 75w90 diff fluid to the engine because thats all they had and the sump was low. They wanted to know if the engine could be damaged. I thought 3 oz wouldn't be enough to hurt, then I thought, What in diff fluid is harmful to engines?
 

MolaKule

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Originally Posted by paoester
Originally Posted by MolaKule
what, if any, is the purpose of comparing engine oils to gear lubes
A practical thread on this very subject came up a while back without any resolution. A person said they added about 3 0z of Redline synthetic 75w90 diff fluid to the engine because thats all they had and the sump was low. They wanted to know if the engine could be damaged. I thought 3 oz wouldn't be enough to hurt, then I thought, What in diff fluid is harmful to engines?
How could anyone tell if an engine was 3 0z. low? It is the EP additive chemistry in Gear Lubes that is harmful to an engine.
Quote
Basic Gear Lubricant Requirements The primary functions of a gear lubricant are to reduce friction and provide cooling. Most of the friction reduction, and certainly the cooling, is provided by the base oils formulated into the gear lube. Secondary functions, such as protection and enhancement of the gear lube, are provided by the additives. Elemental chemicals and synthesized organics provide rust protection, oxidation inhibition, metal deactivation, and increase the Viscosity Index for wide temperature swings. Additional additives for foam suppression, water demulsifiability, and seal compatibility are also important. The most pronounced additives are the anti-wear protection (AW) additives and the anti-galling/anti-scuffing protection provided by the extreme-pressure (EP) additives. Some fluids use Viscosity Index Improvers (VIIs) to broaden the temperature range of operation if the base fluid does not have its own inherent VI. In limited-slip differentials, with clutches based on inertial mechanisms, additional Friction Modification is provided by natural or synthetic esters generally called fats.
Gear Tribology and lubrication II THE primary Extreme Pressure (EP) chemistry is a concentrated sulfur-phosphorus combination that creates acids at combustion temperatures.
 
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OK, makes sense, acid formation. HCl, H2SO4, H3PO4 can form if chlorine, sulfur, or phosphorous get free (heat, stress) and combine with water. Seems like carbonic acid H2CO3 and nitric acid HNO3 are unavoidable though. Maybe weak enough.
 
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I think the point people are making is that gear oil ratings are not the same as motor oil ratings. 80 gear oil is near the viscosity range of 30 motor oil. If they used the same scale, you'd have 10w30 gear oil and motor oil and that would be too confusing for a lot of folks. Some folks think 80 gear oil must be really thick but it's not.
 
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