What does viscosity index really mean?

[TheFuror]

Okay,

I am told that the higher the Viscosity Index number is, the better the oil is at withstanding change due to temperature.

So, I understand this to mean that the oil doesn't crap out when it gets hot and can still pour when cold. If this wrong, please feel free to educate me.

Here is the confusion: many oils I looked at have much better VI numbers for their 5w-30 than for their 10w-30 or 20w-50. Example: M1 5w-30 has a VI number of 167, with 10w-30 of 147. Castrol GTX has numbers which consistently lower the higher the vicsocity is, so that 5w-30 is much better than 20-50 (160 to 128).

Shouldn't the thicker oils have better numbers?

Bob W.

 

[Jay]

Your definition of VI sounds right.

All of the 5w-30s should have had higher VI than the 10w-30s. Viscosity Index is calculated by taking kinematic viscosity at 2 temps; one at 100C, the other at 40C. The less change in viscosity, the higher the VI. 30 weights will have roughly the same viscosity at 100C. A 5w-30 will have much lower viscosity at 40C than a 10w-30--hence the higher VI.

Thicker basestocks generally have better VIs than thinner ones as long as you're comparing apples to apples.

I can send you a kinematic viscosity vs temp chart where you can plot the lines out and see the relationship for yourself if you want. PM me with your email address

[ September 07, 2004, 07:00 PM: Message edited by: Jay ]

 

[TheFuror]

quote:

---

Originally posted by Jay:
The less change in viscosity, the higher the VI. 30 weights will have roughly the same viscosity at 100C. A 5w-30 will have much lower viscosity at 40C than a 10w-30--hence the higher VI.

---

Now I am really confused: if a 5w-30 has a greater change in viscosity, shouldn't it have a lower number?

Bob W.

 

[TheFuror]

quote:

---

Originally posted by TheFuror:

quote:

---

Originally posted by Jay:
The less change in viscosity, the higher the VI. 30 weights will have roughly the same viscosity at 100C. A 5w-30 will have much lower viscosity at 40C than a 10w-30--hence the higher VI.

---

Now I am really confused: if a 5w-30 has a greater change in viscosity, shouldn't it have a lower number?

Bob W.

---

Okay, I get it now: the VI measures the change between the original weight and the viscosity at 40c, right? This is opposed to the change between 40C and 100C, yes?

Since 5w-30 has a lower 40C viscosity, it is closer to the orignal viscosity where they measure the 5w, in the 5w-30, at least I think....

Bob W.

 

[Al]

quote:

---

Originally posted by TheFuror:
Okay, I get it now: the VI measures the change between the original weight and the viscosity at 40c, right? This is opposed to the change between 40C and 100C, yes?

Since 5w-30 has a lower 40C viscosity, it is closer to the orignal viscosity where they measure the 5w, in the 5w-30, at least I think....

Bob W.

---

Yea the 5W-30 will have a slightly higher VI.. Its really a meanlingless number any more with multi vis oils. It can be found by calculation, with a graph or with a chart. But-again - who cares

 

[Jay]

Bob, exactly right. Look at a spec sheet for various 30-weights and look at their viscosities at 40 and 100C. You'll see that the 0w has lower viscosity at 40C than 5w, and 5w lower than 10w, etc.

A high VI is a good quality for any oil to have--especially motorcycle fork or shock oil. Those have VIs in the high 300s and low 400s.