Posted by TheOldHokie on November 28, 2012 at 20:20:35 from (188.8.131.52):
In Reply to: Thanks all! posted by Royse on November 28, 2012 at 18:49:13:
Royse said: (quoted from post at 22:49:13 11/28/12) TOH, would you mind expanding on your answer a little bit and explaining
the meanings of the first and second numbers of multi-vis oil?
For example, what is the difference between say 0W-40 and 15W-40?
Certainly - that's a great question. Too bad Dell repeatedly refuses to listen to the answer. The basic principle is that the grade number tells you what the viscosity of the oil is at a given temperature. And despite the fact it is a number it's not an exact measure but rather a grade label like thin, medium, medium heavy, heavy, extra heavy, etc.
For example SAE 30 means the oil has a viscosity anywhere from 9.3 to 12.5 cST inclusive when measued at exactly 100C. It must also have a minimum high temperature/high shear viscosity of 2.9 cP measured at exactly 150C. This is it's HOT grade number. The SAE defines five HOT grades - 20, 30, 40, 50 and 60. These grades tell you what the viscosity of the oil is when very hot inside a hard working engine (100C and 150C)
There is also a set of COLD grade numbers. They tell you what the oil's viscosity is when it is very cold (-10C to -40C). For example SAE 10W means the oil has a MAXIMUM cranking viscosity of 7000 cP measured at -25C. It must also have a MAXIMUM pumping viscosity of 60,000 cP @ -30C. There are six COLD grade numbers - 0W, 5W, 10W, 15W, 20W, and 25W. Thes grades tell you how the oil will perform on inital startup in a cold engine.
Here is a link to a table that lists all of the SAE grades and their viscosity requirements:
[URl=http://www.widman.biz/English/Tables/J300.html] SAE J300 Motor Oil Viscosity Grades[/url]
Now that we have the meaning of the basic labels defined how are they used and why? The simple answer is refined engine oils are not all alike and we need a "simple" way to categorize them based on their viscosity when both cold and hot. They are refined from different types of crude oil and their physical properties can differ greatly depending on the type of crude, how they are refined, and what additives are blended into them. For example a minimally refined oil made from a high grade crude may be able to meet the SAE 30 hot requirement but can't meet any of the cold requirements. It is a mono-grade SAE 30.
If you subject it to more agressive and expensive refining you may be able to make it meet the SAE 10W standard as well. Now it meets BOTH standards and becomes an SAE 10W30 grade oil.
Similarly you might be able to use that less costly refining process to refine a SAE 10W20 oil from a lower grade crude. And if you blend in some viscosity enhancers to that SAE 10W20 oil you can increase its hot viscosity without affecting it's cold viscosity and turn it into an SAE 10W30 grade oil. Blend in even more viscosity enhancers and you can make it an SAE 10W40 grade oil.
There are limits to how far you can strtch the viscosity band of conventional motor oil. With really agressive refining you can get a SAE 10W20 without resorting to additives. Or possibly a 10W30 or 10W40 with the use of some viscosity enhancing additives. Enter the synthetics. These are custom hydrocarbon chains that are built from the ground up by "gluing" shorter gas (ethylen) molecules together rather than breaking the long molecules in crude up. You have none of the unwanted by products that are in crude oil and you can custom engineer really high performance behavior. A SAE 10W30 with no additives, or a 0W40 with minimal additives. The downside is it's expensive to make.
Hopefully that answers your basic question and I'm guessing creates a few new ones. So here is a link to my long winded explanation:
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