Question regarding Viscosity index

[Baveux]

Can we compare Viscosity index between different grade of oil ? Let say between a 5w30 and a 10w30 ? What tells me that an oil is going to "thin" quicker than another one ? Thanks

 

[G-MAN]

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Originally posted by Baveux: Can we compare Viscosity index between different grade of oil ? Let say between a 5w30 and a 10w30 ? What tells me that an oil is going to "thin" quicker than another one ? Thanks

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Viscosity index doesn't have anything to do with indicating how much an oil will "thin out" with use. The VI is just a measure of how resistent an oil is to change viscosity with temperature change. Generally, the higher the spread between the "w" rating and the upper rating, the higher the VI. Thus, all things being equal, a 5w30 will have a higher VI than a 10w30.

 

[Baveux]

So if I understand correctly I should consider the 5w30 with the higher VI when its the time to choose one. Of course this and some other thing !!

 

[Al]

You should really just ignore it. It has become a really meaningless term. When youy choose a 5W-30 oil- brand x is going to be close to brand y. Viscosity is only measure of the oil's properties. If you decide you want an oil that is relitively thick at operating temperature- you can choose one that is on the high side of the viscosity at 200 F. The additive package is important as well as the makeup of the base oil (Whether it's Group I, II, III, or IV). Most important is how does the oil hold up and protect your engine. That's where this board is useful. You can look at Used Oil Analysis and make a decision based on that. Finally you can run an oil and do an analysis. Then you'll know.

 

[G-MAN]

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Originally posted by Baveux: So if I understand correctly I should consider the 5w30 with the higher VI when its the time to choose one.

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Not necessarily. TooSlick and I have theorized that when it comes to synthetic oils, a LOWER VI compared with other oils of the same viscosity probably indicates the viscosity spread is accomplished mostly with (or totally with) the high VI of the base oil and not with heavy doses of polymeric VI improvers. If this theory is true (and I personally believe it is), then if you were comparing synthetics, I'd go with the 5w30 that had the LOWEST VI.

 

[Ken2]

Viscosity index is calculated by the difference in viscosity at 40°C and 100°C compared to reference oils. Pure Napthenic oil is given the VI of 0 and pure Paraffinic oil is given the VI of 100. I think I'd prefer an oil with a high viscosity index due to top quality base oils, not due to a large amount of viscosity index improvers, and a high 100°C viscosity near the top of the SAE "weight" range for that viscosity grade of oil. Ken

 

[Terry]

You are all on the right track, just an added comment, Vis Index scale is a totally arbitrary and relative measurement that meant something in the "old" pre VII days when motor oil was primarily base oil. When the best oil demonstrated the least rate of vis change from 40c to 100 C , it was given a score of 100. The worst, or oil that changed the most got scored 0. Improved refining and VII render this scale meaningless. [ May 29, 2003, 12:15 AM: Message edited by: Terry ]

 

[satterfi]

I consider viscosity index very important for suspension fluids. Once I know which viscosity I want, I don't want it changing on me. The range available is considerable. On the low end you can get 100 VI for cheap oil and all the up to 400 VI for some good stuff.