Are all 75W-90 gear oils synthetic?

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Ken2

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The owner's manual for a Kia calls for 75W-90 gear oil in the rear differential and the driveline clutch pack/center diff. It doesn't specify synthetic. I'm most interested in low running resistance and fuel savings; the rig isn't run heavily loaded.

Are there any conventional 75W-90 gear oils, or are all synthetic? (I think so, but I'm not sure.)

Is there any real difference between the top brand syn gear oils? I know that Amsoil, Royal Purple, Mobil 1, Red Line and others all have their adherents, but aren't all excellent?
 
Originally Posted By: Ken2
Are there any conventional 75W-90 gear oils,

There are. Valvoline makes one, for example.

Quote:
Is there any real difference between the top brand syn gear oils?

One difference that comes to mind is that some are GL-4 while others are GL-5 rated. If your application specifically calls for GL-4, do not use GL-5. But that mainly matters for manual trans applications. If it's rear diff, it probably doesn't matter too much.
 
Originally Posted By: Rand
what is the application?


Originally Posted By: Ken2
The owner's manual for a Kia calls for 75W-90 gear oil in the rear differential and the driveline clutch pack/center diff.



There are syn and conventional 75W90's. I typically use WalMart Supertech for my refills.
 
Had excellent results after upgrading to Amsoil synthetic 75-90 Severe Gear Oil in a Kia Sorento's front and rear differentials.
 
Quattro: Is the Valvoline conventional 75W90 a fairly new product? Or do you know when it appeared? Not too many years ago, there weren't any conventional 75W90s and they are still rare. Do you know what base oil this one uses? According to the PDS, it has a better VI than Valvoline's syn gear oil and a one degree lower 40C temp rating.

I'm just picking your brain here because this one caught me by surprise. I have a limited slip in one truck that is rated "mineral oil only" pretty vehemently by the mfr. but I would like to get an oil with better cold weather performance than the 80W90 in there now and the Valvoline may be it. The Valvoline is 99cSt@ 40C vs 140 for the Chevron in there now. The Valvoline VI is 166 vs 99 for the Chevron 80W90 installed now.

My first question would be if this Valvoline 75W90 is loaded up with VIIs so that it will shear quickly? Or perhaps is uses a "better" base oil that still can't be called syn? I think they would if they could, so it can't be any better than a GpII+, right?

Anyway, thanks OP for initiating this research project for me and thanks Quattro for giving me something specific to hunt.
 
Originally Posted By: Jim Allen
Quattro: Is the Valvoline conventional 75W90 a fairly new product? Or do you know when it appeared? Not too many years ago, there weren't any conventional 75W90s and they are still rare. Do you know what base oil this one uses? According to the PDS, it has a better VI than Valvoline's syn gear oil and a one degree lower 40C temp rating.

Jim, I really don't know. I just googled conventional 75w-90, and that's how I found this valvoline.
 
Well, maybe that's what Valvoline does, but they will usually brag about any syn content. I'll try the Valvoline hotline first and see if I can get any non-Bozo quality answers.
 
75W-90 lubes seem fairly straight forward. Use the proper GL-# rating. The viscosities can vary and since you are wanting MPGs, maybe you should look for the thinnest 75W-90 you can find?

Other than viscosity, I don't think you'll notice the fluid differences unless you are talking about a manual transmission.
 
I have used the Valvoline 75W-90 conventional in my Tundra front diff and t-case for several years. NAPA has sold it for as long as I can remember. They seem to be tied in with Ashland products by some sort of agreement. I never tested it (UOA), but it performed well over the years and was very seal compatible up until I sold the truck with 207K miles on it. 80W-90 works well too, and is what the Toyota dealers use in all their diff and t-case services. I prefer SAE 90 or 110 in any rear diff though...much more shear stable than the multi-vis stuff.
 
Originally Posted By: INDYMAC
I prefer SAE 90 or 110 in any rear diff though...much more shear stable than the multi-vis stuff.


You would until you went thru a winter. I have straight LE607 in my '86 F250. For the first 10 miles of so on a 'teens day, you don't need the brakes to stop ( : < )
 
Originally Posted By: Jim Allen
Originally Posted By: INDYMAC
I prefer SAE 90 or 110 in any rear diff though...much more shear stable than the multi-vis stuff.


You would until you went thru a winter. I have straight LE607 in my '86 F250. For the first 10 miles of so on a 'teens day, you don't need the brakes to stop ( : < )


Sounds like you're a safer driver now and saving money on brake jobs too Jim. I never thought about those benefits!
 
Originally Posted By: INDYMAC
Originally Posted By: Jim Allen
Originally Posted By: INDYMAC
I prefer SAE 90 or 110 in any rear diff though...much more shear stable than the multi-vis stuff.


You would until you went thru a winter. I have straight LE607 in my '86 F250. For the first 10 miles of so on a 'teens day, you don't need the brakes to stop ( : < )


Sounds like you're a safer driver now and saving money on brake jobs too Jim. I never thought about those benefits!


Well, I've had the LE607 in since '06 I think but the brakes on the truck are still the originals from 1986, believe it or not!
 
A conventional 75W90 will have a good amount of VII which will not protect as well as a pure base oil, especially over time. I would recommend a synthetic gear oil unless you will be driving through water.
 
Originally Posted By: Tempest
A conventional 75W90 will have a good amount of VII

75w-90 gear oil is equivalent to something like 15w-40 engine oil when it comes to viscosity. If they can make a shear stable 15w-40 mineral oil (think most HDEOs), then they should be able to make a shear stable 75w-90 mineral gear oil, too.
 
Quote:
If they can make a shear stable 15w-40 mineral oil (think most HDEOs), then they should be able to make a shear stable 75w-90 mineral gear oil, too.


For sure. They now use shear stable VII's with the brighstocks as thickeners.
 
Originally Posted By: MolaKule
Quote:
If they can make a shear stable 15w-40 mineral oil (think most HDEOs), then they should be able to make a shear stable 75w-90 mineral gear oil, too.


For sure. They now use shear stable VII's with the brighstocks as thickeners.


But if you look around you find hardly any 75W90 mineral oils. Plenty of 80W90 and 85W90. I knew it was possible, especially since VIIs have improved greatly over the last decade or so, but I just didn't see it much. With a cursory Google (as opposed to a detailed Google), you won't find anything but the Valvoline. Perhaps my gobsmackedness stems from cluelessness, but I'm glomming onto somathatthere Valvoline for my F150. The LSD manufacturer calls for mineral but I want that viscosity spread!

Oh, and Mola, that link above goes nowhere.
 
Last edited:
Originally Posted By: MolaKule
Quote:
If they can make a shear stable 15w-40 mineral oil (think most HDEOs), then they should be able to make a shear stable 75w-90 mineral gear oil, too.


For sure. They now use shear stable VII's with the brighstocks as thickeners.


Do these VII provide the same film continuity as the base oil?
 
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