Very stupid question about hot and cold viscoity.

[Johnny248]

I feel stupid for even asking this, but here it goes...

Lets take some oil. I have 5w30 on my shelf, so that will work. If it is rated to flow as a 5w when cold, and a 30w when hot, why does it pour out of the container faster when hot and slower when cold? By the description, shouldn't an oil that is 5w when cold still pour faster than a 30w when hot??? If you take a room temp quart of SAE 30 and pour it, it pours at a certain rate. Take a room temp SAE 50 or SAE 70 and pour it. It is much thicker and pours much slower. This makes sense to me. the 70w is slower flowing than 30w. So, why doesn't this hold true when you pour cold 5w30 vs hot 5w30? It should seem that changing your oil cold would be better than hot because it will leave the engine faster.

All that being said, it really doesn't matter at all but what I am I missing?

I'm sure this is so ridiculous that it should never be on a BITOG posting, but I've always wondered why.

Thanks!

 

[TrevorS]

The first number, in this case 5w is on a different scale than the second number, in this case 30.

Having said that, a 5w20 will likely pour faster than a 0w30 at room temperature, so there is a relationship between the two numbers even though they are not directly comparable.

 

[bvance554]

I don't think its stupid. I've had the same thought myself. Its not intuitive. Could it be because it is flowing at the rate of a hot 30w versus a cold 30wt? Someone will let us know soon.

 

[supercity]

SAE30 is a 30 monongrade. 0w30 and 5w30 are both 30 multigrades or incorrectly referred to as a 30 weights. These are all the same viscosity (within the band for a 30 grade) when hot but different when cold. The "5w" in 5w30 does not mean SAE5 grade when cold, it is only a measure of the properties of the oil when very cold, well below zero.

I'm sure it is explained better in the motor oil university

 

[Kuato]

How about:

Take a straight 30 weight and a 5w30. At a cold temp, the 5w is thinner and pours much more easily.

At operating temp, both have the same flow characteristics.

Fluids in general get thicker when colder.

 

[Kuato]

Darn edit time limitation....

Now take a straight 5 weight vs the 5w30. At a cold temp they would pour the same. But at operating temp the straight 5 weight would be super thin.

 

[RF Overlord]

A multigrade oil, like 5w30, will have the flow characteristics of a 5-grade when cold, and a 30-grade when hot. This doesn't mean it gets THICKER when hot, it means that it doesn't thin out as much as a straight 5-grade would at the same temp.

Arbitrary numbers for illustration:

5-grade oil = 100 when cold and 10 when hot.
30-grade oil = 300 when cold and 30 when hot.
5w30 oil = 100 when cold and 30 when hot.

 

[CharBaby]

I don't think it's about how the oil pours when it's hot or cold, I believe it is about the viscosity that the oil maintains when it is either hot or cold.

 

[jorton]

The viscosity of oil at room temp is much higher than the same oil is after the engine is warmed up.

5w means oil flows at a certain cold temperature.

30 grade means oil has a certain resistance to flow after the engine is warmed up.

 

[Wheels123]

Originally Posted By: TrevorS
The first number, in this case 5w is on a different scale than the second number, in this case 30.




THIS. The two #'s on the bottle don't mean the same thing.

Back when I first became interested in trying to understand oil I was thinking along the same lines you are. Unless someone helps oil can be tricky to figure out. Most people don't have a clue what the numbers on the bottle mean. That's why we're here, no stupid question was asked.

 

[TrevorS]

Study this for a while and you'll get it:

 

[Ken2]

The "W" after the 5...5W...does not mean weight. It means Winter. Table 2 above shows the difference. Any oil viscosity grade with the "W" has passed the cold viscosity tests shown in Table 2. The viscosity grades without the "W" pass the hot viscosity tests shown in Table 1.

So...an oil labeled 5w30 passed the "5W" viscosity grade cold tests and the "30" viscosity grade hot tests. Clear?

 

[TFB1]

Originally Posted By: Ken2
So...an oil labeled 5w30 passed the "5W" viscosity grade cold tests and the "30" viscosity grade hot tests. Clear?


Correct and that 5W oil is probably 20X thicker cold than a hot 30...

 

[Errtt]

So if one mixed a qt of 5 and a qt of 30, would that make it 2-qts of 5-30?

 

[Clevy]

Originally Posted By: Errtt
So if one mixed a qt of 5 and a qt of 30, would that make it 2-qts of 5-30?


Is this serious?
It would make the mixture a light sae 30 grade.
You gotta tell me where you are able to acquire sae 5 grade oil though. I've never seen it before

 

[Errtt]

Originally Posted By: Clevy
Originally Posted By: Errtt
So if one mixed a qt of 5 and a qt of 30, would that make it 2-qts of 5-30?


Is this serious?
It would make the mixture a light sae 30 grade.
You gotta tell me where you are able to acquire sae 5 grade oil though. I've never seen it before

I don't know, just asking. I didn't acquire 5 grade, but some of the light oils I have (like fork oil) whatever grade, seem very light, almost like ATF viscosity.
Was only wondering... thanks

 

[A_Harman]

There is no SAE 5 grade.
There is SAE 5W grade, which is a Winter grade only.
And this is the piece of terminology that many people don't understand about SAE viscosity grading system. There are the W grades, which are intended for low temperature use only, and the standard grades, which are for high temperature use. When the SAE came up with this system, they intended that W grades be used in the winter when maximum daily temperatures never got above freezing, and standard grades were to be used above 32F ambient temperatures. So they came up with 5W, 10W, 15W, 20W, and 25W grades, and 20, 30, 40, 50, and 60 grades. W-grade oils only have to pass the low temperature cranking and pumpability requirements that were posted on the previous page, and also have a minimum viscosity of 3.8cSt at 100C. The standard SAE oils are graded only by their viscosity at 100C.

It wasn't until the early 1950's that oil makers started producing oils that could be used year-round. These are multi-grade oils, and have been tested to meet both the low and high temperature viscosity requirements, and as a result carry designations such as 5w30 or 20W-50. Way back when, you could go into a parts store and buy straight-grade 10W, 20W, 20, 30, and 40 grade oils, and it was expected that the customer would know only to use W-grade oils in cold weather. There were also some multigrades then that sound silly now: 20W-20, 20W-40, 25W-50.

Has anybody out there seen straight W-grade oils on the shelf lately?

 

[Ken2]

Originally Posted By: Errtt
Originally Posted By: Clevy
Originally Posted By: Errtt
So if one mixed a qt of 5 and a qt of 30, would that make it 2-qts of 5-30?


Is this serious?
It would make the mixture a light sae 30 grade.
You gotta tell me where you are able to acquire sae 5 grade oil though. I've never seen it before

I don't know, just asking. I didn't acquire 5 grade, but some of the light oils I have (like fork oil) whatever grade, seem very light, almost like ATF viscosity.
Was only wondering... thanks

You would have a product suitable for dumping into the recycle tank. Engine oil has a very specific package of ingredients (the additive package) that make it suitable for engines, and gasoline engines take a different add pack than diesel engines. As an example, 40 wt engine oil and 90 wt gear oil are actually about the same viscosity but have very different additive packages as you can tell from the smell. ATF has its own add pack for its unique service. Motorcycle fork oil is a hydraulic oil with antiwear and antifoam additives. 10 wt. fork oil is about the same viscosity as ATF, which makes a pretty good fork oil. (And, it was a mistake years ago for fork oil makers to use "weights" for their fork oil viscosity. It should carry hydraulic oil viscosity grades. Oil "weights" are only for engine and gear oils.)

 

[J. A. Rizzo]

This question does not reflect well on the decision outlined in your signature - shockingly so when you also consider your post count. Sorry if that sounds harsh, but this is like fundamental stuff with motor oil. I'm not sure how one could properly evaluate the quality or suitability of a product without understanding the concepts of 1) the W in oil grades and 2) the physical properties the numbers represent. Frankly, I'm shocked at some of the answers you've received as well. There seems to be widespread misunderstanding of some of the most basic properties of motor oil. This is alarming to me, but then I don't spend much time here. Maybe this is why. There's an awful lot of talking from people who really don't know what they're talking about. I might get flamed or banned for this, but to say I'm stunned is an understatement. Kuato and RF Overlord have the best explanations here.

 

[A_Harman]

Originally Posted By: J. A. Rizzo
Kuato and RF Overlord have the best explanations here.


Actually, I thought my explanation was best, but I'm biased.