MFE, I'm well aware. The point I was making was that contrary to the common misunderstanding, a lower "W" multigrade oil is not going to become thinner at lower temperatures.
A 10w-50, 15w-50, 20w-50, all behave like 50-weights at 100C.
yes.
But they behave like 10, 15, 0r 20-weight oils at sub-freezing temps.
Kind of...but not exactly
I hope this doesn't offend you, but that's a very simplistic, and sometimes incorrect, interpretation of a very complex subject. I'll show an example of why, and don't just consider weight because there are other standards that must be met to classify the grade of the oil. If it passes the viscosity/temperature standard, it may still get bumped into another grade because it fails a sheer standard for a given grade.
Under our normal operating conditions, 0W-40 isn't less than 5W-30 at startup. In other words, it is not thinner. Mobile 1's 0W-40 will actually have more resistance to flow in a cold motor than its 5W-30.
Some hard data:
Mobile 1 5W-30 product data sheet
http://www.mobil.com/USA-English/Lubes/PDS/GLXXENPVLMOMobil1_5W-30.aspx
Mobile 1 0W-40 product data sheet
http://www.mobil.com/USA-English/Lubes/PDS/GLXXENPVLMOMobil_1_0W-40.aspx
At 40* celsius (104*F), which is the lowest temp for which their viscosity is listed, the 5W-30 is thinner (61.7 cSt vs. 75 cSt). At operating temperature, as anyone would expect, the 5W-30 is also thinner (11cST vs. 13.5cSt).
This website produces a viscosity vs. temperature graph for each oil if you know the information on the product data sheet:
http://www.widman.biz/English/Calculators/Graph.html
It's broken down in 5* C increments, but for the interest of brevity, I'm not going to list the results of every cell. If you want that level of detail, just enter the data from the product sheet into the flash app at the link above.
Here's the viscosity numbers that it pumped out:
M1 0W-40
Temp (C)/ viscosity (cSt) 0W40/ vicsocity (cSt) 5W30
-35/10804/10939
-30/6402.7/6279
-20/2520/2346
-10/1129/1011
0/563/490
10/307/261.7
20/180.8/151.6
40/75/61.7
60/37.4/30.5
80/21.4/17.4
100/13.5/11
So, the results indicate that the 5W-30 oil is thinner than the 0W-40 at any temperature higher than -35*C (-31*F). At that temperature and colder, 0W-40 is finally thinner than 5W-30. Now, I it gets cold here in Germany, but I'm certain I have never started my corvette at or below this temperature.
This shouldn't be a surprise since you're comparing a 40 weight oil to a 30 weight oil. Now, for what it's worth, I believe from a technical perspective that 5W-30 is the better selection in general at the kind of "freezing" temperatures that I will experience, but it's precisely because the oil is thinner and will flow more easily than the heavier "0W" oil, which I think is contrary to your reasoning. Seems like someone (not you) might argue that the thicker oil offers more protection at low temperature or something, but as you can see above, both oils are much thicker at low temperatures than they are at operating temperatures. So, the oils are obviously more than thick enough to provide adequate protection. More important is how quickly each oil will flow through the lubrication system and build pressure.
In the older days, oil pumps couldn't generate the pressure to move the oil from the pan when it was too cold. That's not going to happen with either of these oils unless you're in antarctica. So, the entire argument is moot. Bottom line: these oils are very close to each other in cold weather performance.
In a racing application, we'd be talking about other things (shear strength, new operating temps, high temp areas of the motor, etc...). Then the advantages of a 40 weight oil might really begin to be significant. But even cruising on the autobahn at 200 kph (124mph) for several hours is not going to begin to push the car like 2 back to back hotlaps at the nurburgring.
Anyway, the point is, there's a lot of things that go into grade. I know you were trying to kind of fill me in on the meaning of the W, but I was aware of the "winter" rating, and didn't want to state something incorrect while at the same time, I wasn't originally planning on giving a class.