One of the properties of engine oil is viscosity. This viscosity is apparently measured per ASTM.
For instance, if you look at Mobil 1's 5w20, it lists the following for viscosity:

Viscosity, ASTM D 445
cSt @ 40º C 48.3
cSt @ 100º C 8.8

Viscosity Index, ASTM D 2270 163

Anyone know what does "cSt" and "Viscosity Index" represent? Does the lower the number = the lower the viscosity? Basically, can anyone describe those ratings in layman's terms?

 

Here's a link that may help to understand viscosity.http://www.astm.org/SNEWS/JUNE_2004/lanhen_jun04.html

 

That's some good information, but it doesn't answer my question.

 

DTS419, to help you with your quest, you are going to have some degree of basic engineering. Even shade tree engineering is helpful. Lets start by defining some of the terminology.
cSt means centistokes (not strokes-stokes) A centistoke is defined as a unit of kinematic viscosity. 1 centistoke= 0.01 stoke. Now that is the definition, so what does that mean? We have to define all of the terms involved. A stoke is the kinematic measurement of a fluids resistance to flow defined by the ratio of the fluids dynamic viscosity to its density. Now we need to define for you what is kinematic viscosity. It is the time required for a fixed amount of oil to flow through a capillary tube under the force of gravity. The unit of kinematic viscosity is the STOKE or CENTISTOKE which is 1/100 of a stoke. Kinematic viscosity may be defined as the quotient of the absolute viscosity in centipoises divided by the specific gravity of a fluid, both at the same temperature. I can only assume that this is as clear as mud- sorry, this isn't an easy subject to understand for most folks. Here is a link that may help you understand better.
http://www.engineeringtoolbox.com/dy...ity-d_412.html

I googled this site for you and it is the clearest and most concise information I could find. I hope you can understand what it means. Making a motor oil isn't just taking a fluid of known viscosity and throwing in a bunch of additives. The lead blenders have been doing this for decades and it takes decades to learn.

 

Sorry, I missed the Viscosity Index. It is defined as a common measurement of a fluids change of viscosity with temperature. The higher the Index number, the smaller the change in viscosity with temperature. As you probably know, as most oils heat up to running temp, they thin. A 5w-30 oil will start out with the flow properties of a 5 viscosity fluid when cold and then when it heats up, it is what a 30 viscosity fluid would be at running temperature. So even though it started out as a 5 viscosity fluid COLD (which means it is thick but still flows where a 30 may not) as it heats up, it will only thin out to the same viscosity as a 30 oil at full running temperature. If we engineer a fluid with properties that will do this without any viscosity enhancers, we don't have to worry about those additives depleting or causing sludge problems like the old 10w-40 oils did back in the 80's. An oil with a higher VI will be more stable in the long run. Hope this is the info you needed.

 

Labnerd,
Thanks very kindly for the explanation. As a matter of fact, I am actually an engineer myself; however, my discipline is structural- not very helpful to engine oil theory.
I went through an old fluid mechanics textbook I had and it vaguely discussed engine oil viscosity in not much more than layterms. I understand the big picture, I just wasn't sure what all the specs actually mean to the consumer that the major oil manufactures list on their websites. Also, I was trying to get an idea of the magnitude of a difference in viscosity rating. For instance, one oil might be listed a cSt @ 100 C = 48 and another might be 63. Common sense alone would say that the 63 is of higher viscosity, but are we talking the difference between rubbing alcohol and maple syrup, or is that difference so small that its virtually undetectable... Anyway, interesting stuff, and thanks again for the info.

 

^^^ Ya what he said

The first set of numbers are a method and measurement of determining flow at the used temperature standards. The second standard I believe relates to the standard that rates the viscosity of oils.

D 2270 Nomograph for reading viscosity index
http://www.flourmilling.net/tables.html#table15

Got no idea what flourmilling has to do with anything