Viscosity index definition

I would guess a Royal Purple guy... Some validity to the statement in a simplistic way. The higher the VI the better performance the oil will have at extreme temps. So at extreme hot temps, depending on the base stock, the oil will not thin as readily. That same oil could have less thickening at extreme cold temps. A good test for cold temp performance is to take a lower quality product and a higher quality product and put them in your freezer for a few days. You will find most synthetics will pour easier than most petroleum. You will find the easier it pours in cold weather the more likely the VI will be higher. However, caution is urged if you are trying to use what you feel as "film thickness". For example, a 5w30 synthetic will likely feel "thinner" than a 5w30 petroleum. Same is true between different quality synthetic stocks.

Randy S,

The first part of the statement you received is correct, because that is exactly what VI identifies. The second part (the one about "less thickening") refers to oil's fluidity at low temp, and it is expressed via Pour Point and not VI.

Although, some oils with high VI have low Pour Point characteristics, there are some petroleum based oils (i.e. naphthenic) with very low VI (60-70) but better fluidity at low temp than some oils with high VI.

I see that Toyota's 0w-20 oil made by Mobil has a VI over 200 so Toyota has speced an oil that has a better VI than Mobil I 0w-20. The Toyota 0w-20 would be a good choice if one could find it at a reasonable price.

quote:

Originally posted by Donald George:
I see that Toyota's 0w-20 oil made by Mobil has a VI over 200 so Toyota has speced an oil that has a better VI than Mobil I 0w-20. The Toyota 0w-20 would be a good choice if one could find it at a reasonable price.

I didn't get OP was asking for a specific viscosity for a specific application. But since you brought these details into the conversation it seems appropriate to bring up viscosity index improvers or VII's. VII'S are obviously used to increase the VI of the new oil to improve the numbers but degrade quicker in operation. So it is more important to know what the inherent VI of the base stock being used. A cursory search for the Toyota 0W20 mentioned in the post, only says its full synthetic but doesn't mention which group. We know Mobil 1 uses PAO synthetics in some quantity in their formulations so we can reasonably assume it is the same with this product. PAO synthetic base has a naturally high VI anyway. However, to buy or recommend an oil based solely on it's VI when new is not a good practice.

VI is the natural property of oil, VI improver (Additive) can only improve the Viscocity index.

For me the best definition is:

"VI is a number that indicates the capacity of the lubricant viscosity in not variate acordly to the temperature."

This number you get making a calcule that includes the viscosity cSt @40ºC and @100ºC.

The higher the VI the best it is. It can be given by the kind of basic oil (synthetic or mineral) and thru the aditive (VI Improver - solid adictive)