I wouldn't argue with you, because I don't know enough to even do so.

However, the first number is followed by a "W" which means Winter, and it is the viscosity when cold. I'm not sure what temp, but I would guess 0°C. Only a guess. The second number is the viscosity at operating temperature. This is what I meant when I misused the term "base stocks".

Here is an explanation simple enough for everyone:
HowStuffWorks "Measuring Motor Oil Viscosity"

"Multi-weight oils (such as 10W-30) are a new invention made possible by adding polymers to oil. The polymers allow the oil to have different weights at different temperatures. The first number indicates the viscosity of the oil at a cold temperature, while the second number indicates the viscosity at operating temperature."

 

I'm not sure if I agree with this logic about cold viscosity vs operating temperature viscosity. If the oil viscosity was thinner at cold temperatures then we would drain our oil during oil changes when the engine is cold instead of warm temps. Am I wrong?

 

All he is saying is that the oil is thinner at a cold temperature than a straight 40 weight would be (not that it is thinner than a 40 weight at high temperatures).

The low number refers to meeting a certain viscosity standard definition at apprx -20 *C or around 0 *F and the higher number refers to the viscosity performance at 100 *C.

There are even more rigorous requirements ....

The Society of Automotive Engineers (SAE) designates engine-oil specifications. The SAE viscosity grade tells consumers at what temperature the oil is designed to operate. This spec defines the properties of W (winter) oil ranging from 0W up to 25W viscosity. The SAE also specifies standards for straight-grade 20- to 60-weight oils. The W grades are tested at low temperatures. The specification states what the viscosity of the oil must be at a given temperature for engine cranking and starting. For instance, a 0W oil has to have a minimum cranking viscosity of 3,250 cP at minus 30 degrees centigrade (C). A 10W oil has to have a viscosity below 3,500 cP at minus 20 degrees. The requirements for cranking the lower-viscosity oil are more stringent.


Multigrade oils is use viscosity index (VI) improvers to change the viscosity of the oil. Viscosity improvers are specially formulated polymers made of long chains of carbon and hydrogen atoms. At extremely low temperatures, they curl up in a little ball.
As the temperature increases, the ***** of atoms expand, straighten out, and become longer. As they become longer, they contribute to the viscosity of the oil. At high temperatures, these long polymer chains spread out through the oil and have a tendency to thicken the oil. There is a physical change to a multigrade oil as it heats up.


As with most things in life, too much of a good thing is bad. Adding too many polymers to a thin-base oil is not good. The wider the range between viscosities, the more polymers that must be added. A 0W40 oil will have more viscosity index improvers than a 15W40 oil. The problem is the polymers break down or are sheared and can no longer increase the viscosity of the oil. They break up as the oil flows through the engine and squeezes through 0.002-inch bearing clearances. They pull apart when sucked through the oil pump. The oil can start as a 0W40, but over time it could become a 0W30 (or lower) because the polymers are physically broken down and can no longer thicken the oil.

 

Well said, Mark.

So actual visocity of a multi-vis oil is an illusive thing, and comparing one to a single weight oil will be frustrating.

The 15W-40 numbers have to be thought of as viscosities relative to single grade oils at specific temperatures.

Using 15W-40 as an example -

The 15W means the oil will be the equivalent of 15 weight at a cold cranking temperature of x°F. Still not sure what temp that would be, so let's just say it is cold.

The -40 means the oil will be the equivalent of 40 weight at a hot temperature of 212°F (approximately engine operating temperature).

So when we compare these to 5W-40, we can immediately see that when cold cranking, the 5W is going to be lower viscosity than the 15W. But when the engine oil reaches operating temperature, they will be the same viscosity, a viscosity equal to what a 40 weight oil would be at the same temperature. Seems like a win-win, except that the polymer additives tend to break down. So even though it may be a synthetic, don't try to extend your oil change intervals.

Just thinking out loud here.

 

Apologies for hijack.... but Bill,
great pic in your signature

started watching Deadwood a few months back and while ROUGH....

good entertainment and imagine it was even rougher in real life !!!!

Hearst is portrayed as a SOB !!!! and nowdays he's worshipped by some

 

Exactly.

You nailed it all down quite well.

One thing to remember is that multi-vis oils came about when group-II base oils were prevalent. They did not "stretch" well, especially compared to today's group-IV base oils. These days, ANY oil will provide baseline lubrication.

As often as not, owners have other reasons to choose than raw viscosity. Soot suspension, heat resistance, cold-cranking ability, TBN retention, etc...

Basically, pick an oil; any oil... change it often.

-blaine