What is Viscosity Index

Not open for further replies.
May 27, 2002
Ocala, Florida
Good question Patman,

Temperatrue is a major factor when discussing viscosity of an oil. Visc has an inverse relationship with temperature Visc decreases with increasing temperature (the oil thins out) and rises with decreasing temperature (the oil thickens). The rate of change of viscosity with temperature change can be dramatic.

Visc's are usually measured at certain standard temps, such as 40 and 100 degrees centigrade or celsius, which have proven useful in the selection of oils for different applications. In use, an oils visc varies depending on the temp at various locations in the lubrication system of the equipment. For example, it would be different in the piston ring area of an engine than in the main bearing area.

So, the most common means of discribing an oils visc/temperature relationship is with an empirical number called the "viscosity index" or "VI". Again, the relationships are inverse. The lower the VI, the greater the change of visc of the oil with temperature.
One thing I've always wondered, exactly what is the Viscosity Index
number you see in some of the spec listings for an oil? I know that
these numbers can only be compared with other oils of the same
viscosity, and that a higher number is supposedly better, but what
exactly does that number mean?
I'm a bit confused when I look at some of the Viscosity Index numbers I see on the technical data sheet, for instance, I see Valvoline 10w30 synthetic (which uses a group 3 base) having a VI of 149, while Redline 10w30 with it's superior base oil, has a VI of only 137. I would expect the opposite, I would think that Redline would have the more favorable number. I also see that Mobil 1 10w30 has a VI of 145, and Amsoil is 167.

Or does having a higher number not necessarily always equal a better oil?
I think you're starting to catch on! There is more to oil than just the base stock which you seem to have been dwelling on about who has what in the synth base.

One of the reasons I made the oil analysis section is so you can really look at numbers on actual used oil so you can see that in some cases that the base stock is not what dictates a good oil but a combination of base stock and additives.

I personally think redline isn't as good as what some say. The thing I'm not impressed with is first the price, and second it seems to carry a higher level of oxidation levels for the same mileage as a competitor such as schaeffers that has the moly like theirs but only uses about 25%pao. I consider redline to be a specialized oil like schaeffers unlike maxlife and mobil. Do you Have you any ideas as to why a blend can hold up to what redline is doing?

As for maxlife, as pointed out before, I'm not comfortable with the disulphate they use and wouldn't use it in my car for that reason alone, but also I'm not convinced yet that it can handle any extended drains because of the additive levels i have seen.

just my 2cents worth.
Last edited by a moderator:
Thanks for your thoughts Bob, one thing I have learned in the last little while, is that I am not quite as certain about which oils are good and which ones are bad anymore, it definitely is a tricky game out there! But you are absolutely right, oil analysis is the key, and I'm suprised I have waited this long to start doing oil analysis, since I really have been interested in oil for well over 14 years now.
I always thought that an oil with a high VI was good. I spent wat too much time trying to find one. I thought i would have higher better oil pressure. Not true. I have found i have the same pressures using high or low VI oils. I am using Valvoline dino now in my Jeep at 135 VI for 10w-30. Castrol GTX was 143. Oil pressures were the same. Look at Redline Vi numbers.
That is where it gets tricky, Redline's oils are supposedly one of the best, yet their VI doesn't look that great compared to others. Redline has to be a better oil than Valvoline's fake synthetic, yet Valvoline 10w30 shows a VI of 149 to Redline's 137. Very puzzling indeed.

Would my oil pressure be the same if i used 5w-20 vs. 20w-50? As long as the 5w-20 had enought viscosity to get up to pressure? Is the oil pressure gauge a good way to indicate the viscosity needed? I hope this makes sense.
Your maximum oil pressure would be whatever your oil pressure relief valve is set at (factory setting).
Hard to say exactly what the pressure would be at various rpms. Incidently, many cars with "oil pressure guages", particulary Fords, really just have a on-off setup, similar to the idiot light.
No way you can tell actual oil pressure as long as it is over a few pounds (unless you put an actual guage in there).
That is where it gets tricky, Redline's oils are supposedly one of the best, yet their VI doesn't look that great compared to others. Redline has to be a better oil than Valvoline's fake synthetic, yet Valvoline 10w30 shows a VI of 149 to Redline's 137. Very puzzling indeed.

That's because it's not just base oils that are the issue. Some base oils have higher inherent VIs than other bases oils. That is probably the case with Redline.

However, VII (Viscosity Index Improvers) are a thing. You can take a base oil with a lower inherent VI, add VIIs and get an oil with a higher VI.

But, like most things in this world you don't get something for nothing. VIIs will generally degrade with time, temperature, and mechanical shearing. So an oil with a lot of VIIs will see it's viscosity index degrade as the oil ages and the miles roll up. Whereas an oil with a higher inherent VI and little to no VIIs will degrade to a far smaller extent.

I don't think Redline puts in a lot of VIIs in there oils (maybe even none). Particularly in their racing oils which will generally not be run over a wide temperature range.

Edit: Didn't see this was a bumped ancient thread. A golden oldie.
Not open for further replies.