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Low VI gear oils #5332068 01/25/20 06:28 PM
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dparm Offline OP
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I'm shopping around for 75w90 gear oils for my C7 Corvette and am noticing that many of the well-regarded or track-focused products (Gear 300 LS, Amsoil SVG, Delvac 1, etc.) have very low VIs...usually between 140 and 160. Why are these so low despite being synthetic formulations?

By comparison, Renewable Lube's Bio-SynXtra 75w90 has a VI of 187, I suspect due to its KV @ 40C being an impressive 90cSt versus the above fluids which are all 100-120. As an aside, I find it odd that the non-LS version of Motul Gear 300 has an insane VI of 222, again due to an even more impressive KV @ 40C being 72.6cSt!

Is RLI's fluid really that much better due to its naturally high-VI basestocks, are they using VIIs, or is there something else at play? At a glance, it seems foolish not to take the RLI.


2017 Chevrolet Corvette Grand Sport
Re: Low VI gear oils [Re: dparm] #5332148 01/25/20 07:51 PM
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SteveG4 Offline
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Good questions! Is thinner better in high-performance vehicles? I don't know and hope for smart answers here. smile

For the last decade, I've stuck with Mobil 1™ Synthetic Gear Lube LS and have enjoyed its performance in cold Alaska.

[Linked Image]




Somewhere between Alaska and Wisconsin...
2006 Silverado LT Crew Cab 4WD & 2013 Equinox AWD
Re: Low VI gear oils [Re: dparm] #5332154 01/25/20 08:01 PM
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OVERKILL Offline
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The VI of PAO base oils is very close to the range you are mentioning, so anything "high" will be pumped up with VII, which I don't think you want. The VI range you mention certainly isn't "low".


2019 RAM 1500 Sport - Mobil 1 EP 0w-20, FRAM Ultra
2020 Grand Cherokee SRT - Ravenol SSL 0w-40, FRAM Ultra
Re: Low VI gear oils [Re: OVERKILL] #5332178 01/25/20 08:33 PM
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dparm Offline OP
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Originally Posted by SteveG4
Good questions! Is thinner better in high-performance vehicles? I don't know and hope for smart answers here. smile

For the last decade, I've stuck with Mobil 1™ Synthetic Gear Lube LS and have enjoyed its performance in cold Alaska.



To be more precise, all of the fluids I mentioned have very similar high-temp viscosities. Your point is certainly valid -- is a rear diff getting as hot as motor oil? Probably not. My car doesn't have a sending unit for temperature in the differential, but given that the car has a transaxle where the MTF, rear diff, and eLSD are all integrated into one big unit, there is a disadvantage with heat soak. The highest transmission temp I've ever seen is ~250 F which was at the end of a 30 minute session in 100 degree ambient temps. It does have dedicated transmission AND differential coolers.

But...the car is not on track all of the time. It all comes back to the "as thin as possible, but as thick as necessary" mantra.



Originally Posted by OVERKILL
The VI of PAO base oils is very close to the range you are mentioning, so anything "high" will be pumped up with VII, which I don't think you want. The VI range you mention certainly isn't "low".


Good info, but I assume RLI's gear oils are group V, like their motor oils. Their marketing folks always use this phrase: "This biobased formula has combined Stabilized HOBS technology with synthetic base stocks to provide a super high viscosity index (VI) with fuel savings performance." This leads me to believe it is not using VIIs.


2017 Chevrolet Corvette Grand Sport
Re: Low VI gear oils [Re: dparm] #5332180 01/25/20 08:37 PM
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OVERKILL Offline
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Originally Posted by dparm
Originally Posted by OVERKILL
The VI of PAO base oils is very close to the range you are mentioning, so anything "high" will be pumped up with VII, which I don't think you want. The VI range you mention certainly isn't "low".


Good info, but I assume RLI's gear oils are group V, like their motor oils. Their marketing folks always use this phrase: " This biobased formula has combined Stabilized HOBS technology with synthetic base stocks to provide a super high viscosity index (VI) with fuel savings performance". This leads me to believe it is not using VIIs.


Assuming they are using high visc POE for example, we could check XOM Chemical:
https://www.exxonmobilchemical.com/en/products/synthetic-base-stocks/esters

Esterex TM111, which is their heaviest Ester product, has a VI of 81:
https://www.exxonmobilchemical.com/en/chemicals/webapi/dps/v1/datasheets/150000000347/0/en

PAO is much higher.

shrug


2019 RAM 1500 Sport - Mobil 1 EP 0w-20, FRAM Ultra
2020 Grand Cherokee SRT - Ravenol SSL 0w-40, FRAM Ultra
Re: Low VI gear oils [Re: dparm] #5332195 01/25/20 08:51 PM
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sloinker Offline
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I would think the nature of a differential is a shear machine in and of itself. Add pack VI are easily sheared if I understand the literature. I would think that a gear oil with natural viscosity from a Group IV or V would be preferable. I know that racing vehicles like in Nascar utilize coolers for their differentials. I wonder what they use? Light to get rid of parasitic loss but able to deal with heat and shear. I'm guessing a robust group IV/V combination.


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Re: Low VI gear oils [Re: sloinker] #5332218 01/25/20 09:12 PM
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dparm Offline OP
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Originally Posted by sloinker
I would think the nature of a differential is a shear machine in and of itself. Add pack VI are easily sheared if I understand the literature. I would think that a gear oil with natural viscosity from a Group IV or V would be preferable. I know that racing vehicles like in Nascar utilize coolers for their differentials. I wonder what they use? Light to get rid of parasitic loss but able to deal with heat and shear. I'm guessing a robust group IV/V combination.


They're using whatever will sap the least power and just barely get the car across the finish line. cool


2017 Chevrolet Corvette Grand Sport
Re: Low VI gear oils [Re: dparm] #5332889 01/26/20 04:55 PM
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nascarnation Offline
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Originally Posted by dparm

They're using whatever will sap the least power and just barely get the car across the finish line. cool


For sure.
I know for Indycar qualifying they use stuff that's about as thick as kerosene, but not for the race.

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