Low VI gear oils

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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.
 
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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 1â„¢ Synthetic Gear Lube LS and have enjoyed its performance in cold Alaska. [Linked Image]
 
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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".
 
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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 1â„¢ 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.
 
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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
 
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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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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
 
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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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