How important is 40 deg. C viscosity?

[addyguy]

When comparing characteristics of various oils, on thing that thick oils are often knocked for is having a very high 40 deg. C viscosity - e, a 5w30 oil is around 58 cst at 40C; whereas a 5w-50 is around 104 cst. I know this shows that as the oil heats up, it get much 'thicker' at this temp on its way to 100C.

One of the reasons this reading is knocked in thicker oils is that in cold temperatures the oil will be much thicker, and may not flow as well. 40C is not hot as far as engine temperatures are concerend, but as far as air temperature is concerned, 40C is very warm. So it is reasonable to say that is an engine is 40C, it will still feel warm to the touch. If this is the case, then surely oil is something that is 'warm to the touch' will be very fluid.

I mean, think about it - you are in a store that is room temperature, about 22 deg. C. you have bottles of oil on the shelf. Even the thickest grades, like 5W-50 or 20W-50, are very fluid at that temp. Look at one of the clear bottles of Quaker State 5w-50 at that temp - it sloshes around perfectly freely. It doesn't look the slightest bit thick. It seems that it is only below 0 deg. C. that you start seeing a real change in oil thickness for almost all grades.

As I'm writing this, I'm thinking that the significance of the 40C number is that if the oil is thicker at lower temps, it will take longer to warm up, so a 5w30 will 'heat up' faster than a 5W-50, so it will burn off moisture and fuel faster, which is important for short-trip usage. Is this all the 40C number is important for? I'm just trying to learn......

 

[Patman]

I really wish that they would give the viscosity of an oil at a slightly lower temp, such as 20C or even 0C (or both!) as I think this is a temperature that makes more sense. The 40c value does give us roughly an idea of things, but a 20c and/or 0c value would be more valuable, IMO.

 

[Gary Allan]

I think it's an antiquated system just due to the vastly broadened VI of oils as they've evolved. Wasn't the system created when single grade oils had VI of something near 100 (96

).

That is, 100VI was a convenient number to use and, more or less, dictated the temp at which you measured it

An oil that has a 100C of 10 CST and 40C of 100 CST has a VI of 100 ..or 10.0X factor.

 

[Shannow]

I agree with Patman.

0C, and 100C would make more sense.

 

[Jimbo]

Gary nailed it. Its just a data point to easily calcuate VI, nothing more.

 

[Tempest]

40C is a relevant # here in Vegas summers.

 

[deven]

The oil always gets thinner as it heats up not thicker.

 

[ewetho]

Normally you get a 5W reading at -35F, 10W reading at -30F, 15W reading at -25F and 20W at -20F.
The 104F(40C) reading gives you and idea of where the oil is at in transition. So a Mojave Desert summer start up what would you have and how they compare. The 212F(100C) reading will let you know what it is like while at operating temps.

Convenient point yes, but would let them know how different a 5W-50 or 40 is from 5w30 while at summer temps.

 

[addyguy]

deven: I'm very aware that an oil get thinner as it heats up, not thicker. That is why 'thicker' is in quotes, b/c I mean thicker according to the ISO number - ie, going from a 5 weight to a 50 weight in the ISO grade. I know the '50' just means it doesn't thin down to more than a 50-weight at operating temperature, but that it is actually thicker at cold start-up.....just to clarify....

 

[XS650]

Quote:

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deven: I'm very aware that an oil get thinner as it heats up, not thicker. That is why 'thicker' is in quotes, b/c I mean thicker according to the ISO number - ie, going from a 5 weight to a 50 weight in the ISO grade. I know the '50' just means it doesn't thin down to more than a 50-weight at operating temperature, but that it is actually thicker at cold start-up.....just to clarify....

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Looks like you are referring to SAE numbers, not ISO.

ISO grade are actual viscoisty in cSt at 40C.

 

[LouDawg]

+1 on what Patman said. For most people in moderate climates, 20C would be more meaningful. Remember, as Dr. Haas reminds us, the problem (i.e. engine wear) isn't that oil "thins" too much as it heats, it's that it "thickens" too much as it cools. I'd much rather know the cST at 20C, 10C, 0C, or even -10C than at 40C.