Gear oil 80W-90 versus 75W-90

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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".
Since my car differential and transfer case also requires 80W-90, I kind of thought... why Toyota din't say 75W-90 if that was so great?
To start, I measured my differential temperature after running 30 minutes in 70 F ambient. Result: 110...117F. For people in the rest of the world that's 43...47C.
I am curious... let's take a look at the viscosities of various gear fluids at this temperature. Not at frozen -50C, not at scorching 100C , but at normal temperature for this application, 40C.
Well... looking at that table I don't think at all that 75W-90 is an adequate substitute for 80W-90.
Personally I went on the Redline 75W-110 route, because that's a synthetic with 40C viscosity closer of what my car manual specifies.

gear oil.JPG
 
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yeah a 80w90 will have high vis at 40C than a 75w which as you say is closer to what temp your car runs at.

BUT for the 30 minutes you are warming up the 75w will have higher flow, faster flow to protect gears and will enhance mileage.

So part of the question is in winter how long to warm up? if you drive only a few miles the oil may never get to 30 or 40C.
 

SoNic67

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BUT for the 30 minutes you are warming up the 75w will have higher flow, faster flow to protect gears and will enhance mileage.
Firstly, I am using still a 75W, it just is 110 instead of 90, because that fits better with my 40C differential operating temperature.

Also... Even if I was using 80W, I am warming it up from 60-70F (20-30C), not from frigid -40F/C. There is no benefit, even at 0C/32F, probably.
 
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That's all good info. Personally I've been putting 75w-90 in everything that calls for 80w-90 and I don't think I'll be bothering to change anything back. I'd wager most vehicles go to their grave at 200k+ with factory gear oil in the pumpkin without many complaints, so gear oil is one I don't tend to get hung up on in normal applications.

All that said I'm sure the Redline will serve you well.
 
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That whole premise is flawed.

you are assuming that the 40c temp means anything.. if you actually needed a gear oil that was that thick you would need 85w140 for towing when its much hotter.

That being said you chose a very good gear oil.
 

SoNic67

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If an open differential gets much hotter than 20C over ambient, IMO there is something very wrong with it.
This is not a transmission with a torque converter that uses oil friction to transfer torque. Those are just some gears splashing in oil.
I think that range around 40C is essential and that's why is given in the gear oil data sheets.
And even if towing, required stronger film oil, then I think that probably the 75W-90, at 40-50C is not sufficient when the design asks for 80W-90 viscosity.
That why I think that the replacement of 80W-90, in synthetic oil, is actually 75W-110. Or just stick with 80W-90, not go for the thinner (at 40C) 75W-90.

Posted the table to make others aware. Everyone one can make their choices.
 
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Firstly, I am using still a 75W, it just is 110 instead of 90, because that fits better with my 40C differential operating temperature.

Also... Even if I was using 80W, I am warming it up from 60-70F (20-30C), not from frigid -40F/C. There is no benefit, even at 0C/32F, probably.
then you answered your own question huh
 
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I imagine the speed in which oil can travel through an orifice at 40°c is rather irrelevant. Like engine oils I think you need to be looking at what happens to the oil between the gears under pressure. Much the same way we look at HTHS in an engine oil as it's visocisty in a 'stressful' situation.
 
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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

I just toss in 75w-140 everywhere and could care less about MPG. Never had a diff/xcase failure or issue when running visc. Warm climate here so step up a grade but maybe not step up two if you're in the northern or mountain blizzard zones.

If you fear the 75w90 in place of the manufacturer recommened 80w90, then just use a 75w110. Redline, Amsoil, Ravenol, Driven... have you covered for 75w110's, because you know, 75w140 is always just too scary thick for snowflake dudes requiring 85 or 90 grades.
 

SoNic67

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90 grade long ago 13.5-24
90 grade today 13.5-18.5 with 110 now covering 18.5-24cst
That's the problem that I try to point. We can't look, for this differentials at the 90. That's meaningless, unless your differential runs at 100C/212F.
You need to look at the 40C/104F spec values.
And indeed the "fuel economy" made those fluids, especially the ones on US market, go to the thinner edge of their allowed range.

because you know, 75w140 is always just too scary thick for snowflake dudes requiring 85 or 90 grades.
As opposed to just trow honey or molasses in there, because you're a tough guy?
 
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I let my differential and transfer case magnets choose the viscosity.

Yeah, I'm a tough guy.

Some of my Toyotas went to 75w85 instead of 75w90, with no change in the components. Maybe the engineers didn't know that the grade existed.
 
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Amsoil 80w90 @ 40C is 120.2 cst
Mobil1 75w90 @ 40C is 120.0 cst

:unsure:

The Valvoline one is really thin, much thinner than the others! And Valvoline claims to be the #1 brand of gear oil, so most people who actually change the gear oil are using the thinnest stuff out there! But remember, most people never change it and end up at the junkard on their factory fill.
 

SoNic67

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Amsoil 80w90 @ 40C is 120.2 cst
Mobil1 75w90 @ 40C is 120.0 cst

:unsure:

The Valvoline one is really thin, much thinner than the others! And Valvoline claims to be the #1 brand of gear oil, so most people who actually change the gear oil are using the thinnest stuff out there! But remember, most people never change it and end up at the junkard on their factory fill.
Amsoil is synthetic. The base is as thin as other synthetics probably (at 40C), so that's why.


People keep saying "I have used 75W-90 and was fine. Well... as e can see not all 75W-90 are the same.
Secondly, if they had a HD truck and they never used it as truck, but an a commuting vehicle... might not matter what's inside.

Some of my Toyotas went to 75w85 instead of 75w90, with no change in the components.
Fuel economy gods made them do it?
Mobil 1 has a 75W85 for Mercedes, with 67.9 cSt at 40C and 11.9 cSt at 100C. Good luck with that.
 
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Be careful. 80w90 comes in both synthetic and non synthetic. I think that is more important than 80w90 vs 75w90. Chevy 1/2 tons spec conventional in the front diff. Chevy 3/4 tons specify synthetic in the front diff. Both spec synthetic in the rear diff. (at least this is how is was with the mid-2000 pickups. ) If you were already assuming synthetic, then ignore this and carry on. If you are planning on playing around with both then get a infrared laser guided heat gun. You'll usually see the temp is lower with synthetic. In any case, check your case magnet and see if there is any diff after your OCI. Good topic. :)
 
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Be careful. 80w90 comes in both synthetic and non synthetic. I think that is more important than 80w90 vs 75w90. Chevy 1/2 tons spec conventional in the front diff. Chevy 3/4 tons specify synthetic in the front diff. Both spec synthetic in the rear diff. (at least this is how is was with the mid-2000 pickups. ) If you were already assuming synthetic, then ignore this and carry on. If you are planning on playing around with both then get a infrared laser guided heat gun. You'll usually see the temp is lower with synthetic. In any case, check your case magnet and see if there is any diff after your OCI. Good topic. :)
My 04 Silverado 1500 is this way. I did mine by the book at 119k right after I purchased it last year. Nothing but engine oil had ever been changed
 
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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

I just toss in 75w-140 everywhere and could care less about MPG. Never had a diff/xcase failure or issue when running visc. Warm climate here so step up a grade but maybe not step up two if you're in the northern or mountain blizzard zones.

If you fear the 75w90 in place of the manufacturer recommened 80w90, then just use a 75w110. Redline, Amsoil, Ravenol, Driven... have you covered for 75w110's, because you know, 75w140 is always just too scary thick for snowflake dudes requiring 85 or 90 grades.
This person absolutely gets it.

Sadly I just lost some respect for Motul, they sell an out of specification 80W-90.

Also, nails the reason I recommend 75W-110 for many XXW-90 truck rear differentials.
 
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SoNic67

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So you want to say that GM specified AMSOIL?
If you look on my table on first post you will see AMSOIL 80W-90 there. But I know of no factory fill synthetic 80W-90 like you stated.
Yes, Chevy, GM even Ford have 75W140 synthetic rear axle, but that's not the subject of this thread.
 
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