Oil "weights"....I'm still confused & would appreciate help

Status
Not open for further replies.
Joined
Jan 6, 2005
Messages
9,817
Location
Alabama
I have a reasonable understanding of what all the numbers mean on an oil bottle, but I'm still not clear on a few things.

Here's what confuses me; correct me if I mis-speak:

With a 0w30 oil, the oil must have the cold flowing properties of a "0" weight oil at very cold temperatures, & the performance characteristics of a 30 weight oil at operating temperatures. Right so far?

When referring to 0w-XX oils, I commonly see people saying things such as "The oil is thin when cold & thickens when hot".

Isn't the exact opposite true? If I pour GC at -20°F, it's going to be thicker [pouring] than it would be at 80°F.

So, the best scenario I've been able to come up with in my thick wooden skull is this:
Even though a 0w30 oil is thicker [pouring] at cold temps than at warm temps, it would be even thicker [pouring] at cold temps if it were a 5w-XX, 10w-XX, etc.

Lastly, isn't it possible for a 0w-XX, 5w-XX, & 10w-XX to all be the roughly same thickness at say 80°F? Don't the "W" numbers only come into play when the temps are extremely cold?

Thanks in advance for your replies.
WW
 
quote:

...in my thick wooden skull

Just don't ever admit this to your wife!
grin.gif


Seriously, WW, I shy away from using the term "thin" in any discussion about viscosity.

When describing a 0W oil, the phrase "thickens at a lesser rate" is a more accurate description.

Yes, the 0w30 grade only desribes viscosity at 2 data points, -35C and 100C. A 0w30 is commonly described as a "thin" oil, when it actually is the same or thicker,(GC as an example) than most 5w30's at temps above 32F.

This Viscosity Calculator is still the best method to compare oils & demonstrate actual viscosities at various temperatures.

Use module #4 and input 100C, the 100C viscosity from the oil product data sheet, the VI index, and just pop in a different temperature, -10C, -5C, 0C, etc. to chart the viscosity in cSt.

Note - This calculator also demonstrates that the VI index listed on the product data sheet is simply the slope of the temperature/viscosity graph line from 40C to 100C.
 
Umm, the "W" or winter rating denotes the oil passed a specific cold-cranking test, nothing more. It is unclear to me if it is actually a visc test, but I don't think that is a usefull way to describe it. It just ONE data point, it means nothing about how it thickens at temp, just that it passes the test that is at far below zero, -20f, -25f etc. Some 0w- oils are thicker at say +10f or +30f than 10w oils.
 
quote:

The oil is thin when cold & thickens when hot".

This is how everyone mis-expresses the concept. So much so that they give the impression that they don't know what they're talking about.

It should be correctly stated "thickens less when cold and thins less when hot"...but almost 100% of the time you'll hear someone say, "It's thicker when it's hot" ..as a stand alone statement...leaving out "..when compared to lighter oils at that temperature".
 
AEHaas. In your oil chart you have Castrol's
Syntec 0w30 viscosity at 10 for 100 deg C. The
HT/HS viscosity at 150 deg C is 2.95. I thought
the 0w30 (GC) had a viscosity of 12.1 at 100
and it's HT/HS vis. at 3.6.
confused.gif
 
Rather than the simple starting explanation, and in addition to the other good explanations,

It could be simply stated this way: the oil is designed to behave like a thin oil (for example a straight 0 weight, if there is such a thing) when cold, and to behave like a thicker oil (for example a straight 30 weight) when hot. I understand the goal of this behavior is so that the oil will stay as close to the optimal viscosity as possible throughout the range of temperatures it will experience.


On second thought, it's probably clearer to say it the way Steve S did.
 
quote:

Umm, the "W" or winter rating denotes the oil passed a specific cold-cranking test, nothing more. It is unclear to me if it is actually a visc test,

Yes, that is a dynamic viscosity test. Timing hot oil running through an orifice is a kinematic viscosity test. The dynamic test is used for "W" grades and high temp/high shear viscosities. The kinematic test is used at 100°C for the "30 wt", etc., tests. There is preliminary talk of getting away from the kinematic test because it doesn't do the best job of describing how the oil functions in a hot engine.


Ken
 
Status
Not open for further replies.
Back
Top Bottom