In a practical sense for the oil end user, it is useful to use the Vis @ 40 degrees C, FOR MAKING COMPARISONS BETWEEN DIFFERENT OILS:

For example, two of my favorite oils are Redline 10W30 and 10W40. I have had people argue with me in here that they both have the same viscosity at a cold startup, because the first number is a "10" for both oils. Yet...when you look at the cSt @ 40 degrees, the 10W30 gets a 70 and the 10W40 gets an 88.7. Clearly there is a big difference!!!

I always strive to stay in the MIDDLE of the range of viscosity that my owner's manual recommends for whatever outside temperature, and not push the extremes of either cold or hot. While 10W40 has the best all-around temp range in most of the country, when it gets over 90 degrees F I would go with maybe a 15W50, and when it gets around freezing at least a 10W30 or 5W30/0W30.

Since you brought up "viscosity index".....another pitfall is to get mislead by an oils viscosity index (VI). What manufacturers publish is the VI AFTER adding viscosity index improvers (VII's). (These are little plastic polymer threads that curl up when cold and uncurly when hot...helping to create a multi-viscosity oil. A great idea, but they are also the most degradable part of your oil's additive package...and they tend to linearize inside a bearing!) A savy buyer will look for oils that have a NATURALLY HIGH VI, without the addition of VII's. Here, the ester synthetics are once again the best chemistry!

Folks should note also that the bigger the difference between the two numbers on a multi-viscosity oil, the more VII's it took to make it! For example, to make a 5 weight oil a 5W50 will take a much huger amount than to make a 5W30. Picking an oil where the two numbers are closer toghether, will give you a more "durable" oil.

 

And what I am trying to tell you is that it is not.

If two different oils have a viscosity of 70 at 40 C...
This does NOT guarantee they will be the same viscosity in freezing temps.
It would have to have the same viscosity index also (rate of change).

For instance, you listed 70 and 88.7 @ 40 C.

Now here is a listing of a few SAE 30 oils at 40 C

Castrol = 82.6
Valvoline = 86.45
Havoline = 80.8

Now Mobil 1 0w40.... 78.3


Now since we already know that SAE 30 is not safe to use in freezing temps....
And it doesn't get better than 0w...


Well, I guess you can see why you can tell nothing from the viscosity @ 40 C.


And the people telling you your two oils 10w30 & 10w40 have the same viscosity at cold start up... well, they are pretty much correct... especially from the same brand.

Bottom line is that it's the same base stock... but with more VII's in the 10w40 to keep the viscosity the same as an SAE 40 at 100 C.


In fact, most 5-10-15-20-25w's are going to be similar to another 5-10-15-20-25w at cold startup.

Here is the SAE's J300 they use for "w" classification.



That one is a few years old and it may have been updated, but you get the idea.

And from that, it is hard to tell how cold you can go with each.
All you can really tell is that a smaller number is better.

This is why I check consult my manual and also do freezer tests.
Tells me more about actual condition of the oils in freezing conditions.

 

....and I'm telling you, that the VI becomes a bogus number once they add a bunch of VII's! VII's effect the oil when it is hot, but they do nothing to improve it when cold....so a high VI means nothing on a cold start.

Your example of "Mobil 1 0w40.... 78.3" just serves to illustrate my point that you will be fooling yourself if you just use the first number to determine your oils suitability for a cold start. The only way to not get fooled, is to look at the cSt @ 40 degrees C! A 0W anything should do better then a 78.

(Just for the record, Mobil 1 is now Group III and no longer PAO, as it was a couple years ago! Group III VI's aren't nearly as "naturally high" as either PAO or ester. So, you know it took a ton of VII's to get this 0 weight base oil up to where it will make 40 weight when hot. So if this is your favorite oil.....)

 

You have it right, but then half of it twisted around backwards in your own mind.

You do have the base oil, and it's viscosity changes based on temperature.
How much, depends on quality.

Then you do have the VII's to uncoil and hold the viscosity up at temperature.
Good and bad in that for sure....

The 20/30/40/50 number after the "w" is determined at 100 C (212 F)
An SAE 30 or 10w30 will be about 10 or so... etc.
SAE 30 or synthetic 10w30 should hold there longer (less VII's).

As you can see above, your 40 C viscosity on your Redline multi-vis turns out to be 70 & 88.7.

All the SAE 30 I posted above is also in that range at 40 C.

Castrol SAE 30 = 82.6
Valvoline SAE 30 = 86.45
Havoline SAE 30 = 80.8

Redline 10w30 = 70
Redline 10w40 = 93

As you can see, this viscosity has nothing to do with freezing temps.
It's 104 F!!!!

Redline 10w40 is thicker than SAE 30 at 40 C... and it should be.
Redline 10w40 is thinner than SAE 30 at 0 C... and it should be.

Redline 10w30 and 10w40 are BOTH 6500 @ -25 C
CCS Viscosity, Poise, @*C: 65@-25

This is passing for 10w classification (as it should be) but not thin enough for 5w.
SAE 30 does not pass at all... it's probably like honey @ -25 C.
Redline is safe at say 32 degrees, SAE 30 is not.


Bottom line is, the Redline 10w30 and 10w40 are likely the same oil, except the 10w40 has more VII's to uncoil and make the viscosity higher at 40 C and also at 100 C (so it can be classed as SAE 40 at temp).



Hopefully you are able to now straighten out that one half you have backwards in your mind.
I don't see what's left not to get from the above.


40 C viscosity is only used in conjunction with 100 C viscosity...
Only for the purpose in measuring the rate of viscosity change...
For the purpose of assigning a viscosity index....

100 C viscosity is used to determine SAE grade at operating temperature.
SAE 20/30/40/50... or XXw20/30/40/50


Viscosity at freezing temps is determined by this testing criteria to assign...
0w 10w 15w 20w 25w ....



For instance,
0w can be no more than 6200 @ -35 C
5w can be no more than 6600 @ -30 C
10w can be no more than 7000 @ -25 C


And Redline 5w50 is...
20.5 @ 100 C ... (passing for SAE 50)
128 @ 40 C ... (viscosity index of 185 as a result)
6000 @ -30 C ... (passing for 5w)

And Redline 10w30 is...
10.7 @ 100 C ... (passing for SAE 30)
70 @ 40 C ... (viscosity index of 142 as a result) *more change*
6500 @ -25 C ... (passing for 10w) *thicker at a warmer temp than 5w50 despite being thinner @ 40 C*

 

Again...this is what it is good for, a quick and easy way to compare different oils for the end user!

All one has to do is look in our owner's manuals to tell which oil to run for the current temperature. Beyond that, comparing the Vis @ 40 will tell you which of the different oils out there will have the lower viscosity when cold. For example, a petroleum 10W40 gets a 110, and an ester synthetic 10W40 gets an 88...pretty darn simple to see which one to go with!!!

 

And I have just shown you TWICE...
That 3 random common SAE 30 petroleum oils with LOWER VISCOSITY @ 40 C.

Here they are a 3rd time

Castrol SAE 30 = 82.6
Valvoline SAE 30 = 86.45
Havoline SAE 30 = 80.8

Ester Synthetic 10w40 of yours = 88.0

So now you want to tell me that SAE 30 will have lower viscosity than your ester synthetic when cold????

Again, you do realize this is 40 degrees Celsius right? (104 F)

 

And you do realize that every...

10w30
10w40
10w50
10w60

Must be less than....
7000 @ -25 C

But not less than...
6600 @ -30 C

Else it would be a 5w.


So various brands, types and blends of 10wAnything are going to vary a little in cold viscosity... but not outside the above ranges... else it would be rated a 5w or 15w COLD.

An ester synthetic is just going to be "better" because it can pass that test at -25 C and also pass the test at 100 C for SAE 40... with less VII's. It will stand up to HEAT better probably and stay in grade longer. It's NOT going to be vastly thinner at -25 C.

A 10w40 ester synthetic is not magically going to be thinner than another at -25 C just because it's an ester, or just because it is thinner at 40 C.

If it was that much thinner, it would be rated 5w40... like Rotella...
(which breaks down like hell in my opinion)

 

We no doubt lost the original poster and everyone else...long ago!


For example, my three favorite winter oils:

Redline 10W30...70

Maxima 530MX (5W30)...52.9

Maxima Ultra 0W30...46.8


Summer oils:

Redline 10W30...70

Redline 10W40...88.7

Redline 20W50...138

Redline 20W60...185! (for that old hot running Harley)


Another petroleum vs synthetic example:

Redline 20W50 (polyol ester)....138

Maxima Premium 20W50 (petroleum)....162.91


Dude...are you so blind that you cannot see the trends there???? Do you need a 3rd grader to point them out???

While some of the points you make are technically correct...this is how these numbers can be used in a practical sense by the average guy who is comparing oils. Look past the trees, and you may very well discover the forest!

 

While I don't recall reading this anywhere, it makes sense that 40 degrees C was selected because it is the lowest temp just before the VII's start to unfold. Once they unfold, the "normal physics" one would expect to see (relationship of viscosity vs temp) doesn't apply anymore....like it sould while they are folded. But...clearly the presence of VII's have some effect on things, even when folded and the oil is cold.

Other issues....VII's have no lubricating ability of their own (neither do detergents!), they linearize inside bearings, and are the most degradable component in an oil's additive package. All the more reason, to select synthetics that have naturally high VI's, and need only minimal amounts of VII's. Plus, ester synthetics are polar so they have natural detergent properties, so they require less detergents. Some clear trends emerging here, on what is the best oil chemistry!

 

Things are not always what they seem! Take your example:

Castrol SAE 30 = 82.6
Valvoline SAE 30 = 86.45
Havoline SAE 30 = 80.8

These numbers seem to defy the rules according to you, but these are "straight weight" oils not multi-vis...aren't they! STRAIGHT WEIGHT OILS CONTAIN NO VII'S, SO OF COURSE THEY ARE NOT GOING TO BEHAVE LIKE A MULTI-VIS OIL...are they!!! So, your examples and logic are clearly bogus!!!

I'm a professonal scientist and supervise graduate students doing medical research. I have seen every type of misinterpreted data and attempts to manipulate data to try to support an irrational hypothesis in the book. You're not going to slip this one past me!

Another lesson to learn, is that just because something doesn't make sense to you, that doesn't necessarily mean it isn't the truth! It does however mean that it deserves further analysis. At first I was puzzled with your example above, as you clearly were as well. But after a little thought, it was clear that you are ignoring the absence of VII's.....