When a 0W-30 is not a 0W-30

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twX

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Hey everybody. First post. I've really been geeking out on motor oil for the last couple of weeks, and in the process I've been looking at viscosity specs of different oil brands.

After looking at the specs for pretty much all of Mobil, Amsoil, and Valvoline passenger car oils, I've come to the conclusion that the XW-XX rating of the oils really doesn't mean a whole lot. For instance, Mobil 1 0W-30 is essentially the same viscosity as Mobil 1 5W-30 at all temperatures. Many 5W-30's from other manufacturer's are actually thinner than the Mobil 1 0W-30 at low temps (0 degrees C). Why would they advertise it as a 0W-30? Likewise, Mobil 1 5W-20 Extended Performance is thinner than some 0W-20's at low temps, but with the same high temp viscosity.

The ability of an oil to resist thickening as it cools from the operating temperature is can be measured and is called the viscosity index. For two oils with the same viscosity at high temp, the one with the higher viscosity index is better since the oil will be thinner and easier to pump when it is below full operating temperature.

One would expect wide viscosity range oils such as 0W-40's to have a very high viscosity index, and oils such as 5W-20's to have a lower viscosity index. This is not what is observed, however. The 0W-40 will be significantly thicker than the 5W-20 at low temps, and at high temps it will generally be on the thin side of what is allowed for a 40wt. I suspect that the oil manufacturer's must do this because they just cannot make an oil with a high enough viscosity index to be a "true" 0W-40.

I'm sure many people on this forum are aware of these facts, but I found them interesting so I thought I'd share. Moral of the story? Before choosing an oil, check the viscosity ratings, which are usually available on the manufacturer's websites. And don't expect that 0W-40 oil to flow as well as a 5W-20 on those cold winter mornings.

Cheers
 
Originally Posted By: XS650
Please post the actual 0W and 5W pumping and cranking viscosities for the oils you mentioned.


Here are some viscosities at 0 degrees C, 40 degrees C, and 100 degrees C, all measured in cSt, followed by the viscosity index:

Mobil 1 0W-30: 520, 63.1, 11.0, VI=168
Mobil 1 5W-30: 531, 64.3, 11.3, VI=169

The 5W-30 is slightly thicker at all temps, but these differences are vey small compared to the differences between oils within the same grade. Notice the 5W has a higher VI.

Amsoil 0W-20: 389, 49.1, 9.0, VI=165
Mobil 1 Extended 5W-20: 361, 47.4, 8.9, VI=171

The Mobil is thinner at low temps despite the fact that it's rated 5W.

Mobil 1 0W-40: 601, 80.0, 14.3, VI=187
Castrol Syntec 10W-30: 454, 59.6, 10.1, VI=157

Even with a VI of 187 (highest I've seen) the Mobil 1 0W-40 is significantly thicker than a 10W-30 at 0 degrees C, and much thicker than a 5W-20, though to be fair, the Castrol is thin for a 10W-30.

These viscosities are available on the websites of the respective oil companies with the exception of the viscosities at 0 degrees C, which were calculated based on the published viscosities and VI's.
 
define low temperatures. A 0w- oil is qualified at one temperature (-30 C, IIRC), whereas a 5w- oil is qualified at a different temperature (-25C, IIRC).

While one oil may flow OK, or even flow decently at a lower temperature, generally it has to do with the qualification of the oil at those super-low temperatures.

By the time you get to 0C, or even -10C, for example, the oils that remain "thinner" at the far lower temperatures may have to "adapt" in order to meet the operating viscosity. Remember, viscosity is inverse to temperature for the most part. In order to retain certain other characteristics, it may need to retain viscosity a bit differently in order to meet two targets that could be 130C apart.

Think about it this way, an oil that is qualified at -25C, and has a set spec for viscosity at 100C is easier to get than an oil that must flow properly and is qualified at -30C, and then has to hit the same viscosity bogey at 100C. The spread is bigger, so there is more trade space for things to vary. Its all about how one engineers the parameters to, for lack of better terms, set the slope of the curve between various points.

So you need to consdier how the oil flows and pumps at the spec temperatures for the test. It tells you something about the oil's performance, but Id say that you may be correct... the golden bullet may not be to have both numbers be as low as possible... or at least for all temperatures.

This is part of the reason why many of us gripe about the dumbing down on auto manuals, and the removal of viscosity-temperature plots for most engines...
 
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Originally Posted By: twX
After looking at the specs for pretty much all of Mobil, Amsoil, and Valvoline passenger car oils, I've come to the conclusion that the XW-XX rating of the oils really doesn't mean a whole lot. For instance, Mobil 1 0W-30 is essentially the same viscosity as Mobil 1 5W-30 at all temperatures. Many 5W-30's from other manufacturer's are actually thinner than the Mobil 1 0W-30 at low temps (0 degrees C).


Welcome to the board!

0 degrees C is not a low temperature. By definition, 0W meets a viscosity specs at -35C and -40C, 5W meets them at -30C and -35C, 10W meets them at -25C and -30C, etc. If your climate does not approach those temperatures, then there will be little difference between XW-30s.
 
I listed the viscosity at 0 C since I thought that they were tested at this temp. Turns out they're all tested at 0 Farenheit (-17.8C) for the W rating, according to google. This doesn't really change my conclusions, though.

I calculated the low temp viscosity using something like this:
http://www.geocities.com/CapeCanaveral/3655/VI.html

It won't work for really low temps close to the Pour Point of the oil, but I'm assuming it's still valid at -18C.

Can you link to any info on the temp requirements you listed?

Thanks,
 
Originally Posted By: rpn453
Here's the SAE J300 chart that defines SAE viscosity ratings:

http://www.infineum.com/information/viscosity.html
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The XW doesn't reveal the cSt viscosity. It is entirely possible for a 0W-30 to have a higher cSt viscosity than a 10W-30. Viscosity index can only be compared within same grades. You can't compare the VI of a 5W-30 to a 10W-30. The 10W-30 of equal quality would have a lower number but a better range. The sretch from 5W-30 is more than the stretch from 10W-30.

Play with this:
http://www.widman.biz/Seleccion/Viscosidad/Conversiones/Graph/graph.html
 
Originally Posted By: twX
I listed the viscosity at 0 C since I thought that they were tested at this temp. Turns out they're all tested at 0 Farenheit (-17.8C) for the W rating, according to google. This doesn't really change my conclusions, though.

I calculated the low temp viscosity using something like this:
http://www.geocities.com/CapeCanaveral/3655/VI.html

It won't work for really low temps close to the Pour Point of the oil, but I'm assuming it's still valid at -18C.

Can you link to any info on the temp requirements you listed?

Thanks,


Maybe all oils are tested at 0F or 0C, but that is not the temperature for which the 0w-, 5w-, 10w- and 15w- are set by. See above.

Also, how am I supposed to know that the fit from an internet calculator is correct? What if it assumes a linear fit, but in reality it acts in a nonlinear way through some or all of the temperature range. Id be very wary of calculators which may or may not have basis.

You need to fully understand the methods of determining the oil's two viscosity numbers, and what the tests mean, how they are performed and what the methods infer.
 
Firstly
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SAE J300 is the governing spec for the viscosity ratings. The viscosity calculator you referenced is interesting but has nothing to do with the xW rating. If you want the xW rating for an oil, you just about need to get it from the oil company.

The actual tests are done at different temperatures as someone listed in an earlier post, they are far below 0

As a matter of historical interest, the xW rating was determined at 0F 40 or 50 years ago.

Extrapolating much beyond the highest or lowest temperature that you plug into that VI calculator is iffy at best.

Unless J300 has changed their requirements, an oil company is not allowed to miss-rate the xW viscosity, either to a better or worse value, so an oil that passes the 0W-xx tests would not be allowed to be called a 5W-xx oil if they are registered as API approved.
 
The only way to know the viscosity at a particular temperature is to measure it. Calculating it only gives a ballpark figure, since it's a non-linear function.
 
twX,
Nice work thus far. Keep up the good work. You'll give us all something to discuss and maybe ever argue over. Do all oil companies test their oils at the same temps?
 
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