What is Viscosity Index?

[BlakeB]

I've been looking around at a lot of different oils and ATFs. I think I understand the relationship of viscosity and temperature as it relates to the viscosity index.
I have always thought that a higher viscosity index was better and would flow better than a lower viscosity index fluid, given the same circumstances.

I've been looking at Dex VI ATFs and most all of them have a viscosity index around 150, but if you go to a TES-295 fluid or a clone of it, you'll see a viscosity index around 200.

I thought maybe this was because of the viscosity improvers that different blenders use or maybe somewhat dependent upon the base fluid used.

But then if you go look at Citgo Quatrasyn which uses no viscosity modifiers, you'll see a lower viscosity index at 171.

The Viscosity at 40°C and 100°C aren't too far from each other on any of the mentioned ATFs. I do notice that ATFs with a higher viscosity index generally tend to have a lower Brookfield Viscosity.

I guess I'm asking what the viscosity index really means in practical terms?

Is it something that should be payed attention to in selecting the "best" fluid for your application.

I know that any fluid recommended/approved for a given application will most likely work just fine, that's not what I'm here to debate.

I just want to know what to look for if you're nitpicking for "the best" fluid, after all.... isn't that what we do here?

 

[MolaKule]

Originally Posted by BlakeB


I have always thought that a higher viscosity index was better and would flow better than a lower viscosity index fluid, given the same circumstances.



Most ATF's set the VI by the combination of different viscosity base oils AND the additive viscosity AND the VII component, usually a PMA component. Practically speaking, the set of base oils and additive are mixed, the resulting VI is determined, and then the VII is added to a level to meet the various specs.


Originally Posted by BlakeB

I've been looking at Dex VI ATFs and most all of them have a viscosity index around 150, but if you go to a TES-295 fluid or a clone of it, you'll see a viscosity index around 200.

I thought maybe this was because of the viscosity improvers that different blenders use or maybe somewhat dependent upon the base fluid used.

But then if you go look at Citgo Quatrasyn which uses no viscosity modifiers, you'll see a lower viscosity index at 171.



The Citgo Quatrasyn is basing its VI on the sole contribution of the mix of different viscosity synthetic base oils.


Originally Posted by BlakeB

I guess I'm asking what the viscosity index really means in practical terms?



For ATF's select a fluid that meets the 100C viscosity and the Brookfield viscosity specified for your transmission.

Worrying about one single component of a complex fluid will drive you batty.

 

[RayCJ]

Originally Posted by BlakeB
I've been looking around at a lot of different oils and ATFs. I think I understand the relationship of viscosity and temperature as it relates to the viscosity index.
I have always thought that a higher viscosity index was better and would flow better than a lower viscosity index fluid, given the same circumstances.

I've been looking at Dex VI ATFs and most all of them have a viscosity index around 150, but if you go to a TES-295 fluid or a clone of it, you'll see a viscosity index around 200.

I thought maybe this was because of the viscosity improvers that different blenders use or maybe somewhat dependent upon the base fluid used.

But then if you go look at Citgo Quatrasyn which uses no viscosity modifiers, you'll see a lower viscosity index at 171.

The Viscosity at 40°C and 100°C aren't too far from each other on any of the mentioned ATFs. I do notice that ATFs with a higher viscosity index generally tend to have a lower Brookfield Viscosity.

I guess I'm asking what the viscosity index really means in practical terms?

Is it something that should be payed attention to in selecting the "best" fluid for your application.

I know that any fluid recommended/approved for a given application will most likely work just fine, that's not what I'm here to debate.

I just want to know what to look for if you're nitpicking for "the best" fluid, after all.... isn't that what we do here?



Look at this way... The whole idea of multi-viscosity oil is to have a liquid that stays at an acceptable level of fluidity over a range from cold to hot temperatures. Ideally, such a substance would maintain exactly the same viscosity over a wide range of temperatures... It would always pour the same no matter what the temperature. Oil with a high viscosity index behaves closer to that ideal goal.

 

[BlakeB]

Originally Posted by MolaKule
Originally Posted by BlakeB


I have always thought that a higher viscosity index was better and would flow better than a lower viscosity index fluid, given the same circumstances.



Most ATF's set the VI by the combination of different viscosity base oils AND the additive viscosity AND the VII component, usually a PMA component. Practically speaking, the set of base oils and additive are mixed, the resulting VI is determined, and then the VII is added to a level to meet the various specs.


Originally Posted by BlakeB

I've been looking at Dex VI ATFs and most all of them have a viscosity index around 150, but if you go to a TES-295 fluid or a clone of it, you'll see a viscosity index around 200.

I thought maybe this was because of the viscosity improvers that different blenders use or maybe somewhat dependent upon the base fluid used.

But then if you go look at Citgo Quatrasyn which uses no viscosity modifiers, you'll see a lower viscosity index at 171.



The Citgo Quatrasyn is basing its VI on the sole contribution of the mix of different viscosity synthetic base oils.


Originally Posted by BlakeB

I guess I'm asking what the viscosity index really means in practical terms?



For ATF's select a fluid that meets the 100C viscosity and the Brookfield viscosity specified for your transmission.

Worrying about one single component of a complex fluid will drive you batty.

A lower Brookfield Viscosity is fine, correct? From what I understand, Brookfield Viscosity is just a measure of pumpability at a given temperature?

 

[MolaKule]

essentially, yes.

If you live north of the Mason-Dixon line you might need a low Brookfield viscosity.

The BV is defined as a "low-temperature, low-shear-rate viscosity test of automatic transmission fluids, gear oils, torque and tractor fluids, and industrial and automotive hydraulic oils."