copied this off of a Vette forum, interesting read
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High-Temperature/High-Shear (ASTM D-4683)
This test is a simulation of the shearing effects that would occur within an engine. In fact, it's actually designed to simulate motor oil viscosity in operating crankshaft bearings. The D-4683 is measured by the Tapered Bearing Simulator and simulates motor oil stress at temperature extremes.
You might recall that multi-viscosity petroleum oils tend to use long chain polymers to "beef" themselves up. Under high stress conditions where shearing can occur, these polymers break down.
As they do, the viscosity of the oil decreases. This is what the High Temperature/High Shear test checks for.
The HT/HS test is measured in Centipoise (cP) as the Cold Crank Simulator test is. However, in this case, because you're hoping for the least loss of viscosity with an increase in heat and stress, you want the cP value to remain high (at least relative to the minimum set by the SAE J300 standards).
Each SAE multi-viscosity grade has a specific lower limit for the HT/HS cP value. If a multi-viscosity oil cannot achieve a cP value above that limit, it cannot be classified under that viscosity grade. For instance, according to the SAE J300 specifications, an oil must achieve an HT/HS cP value of 3.7 or higher in order to be classified at the 15w40 viscosity grade.
So, whether this data appears on a manufacturer's tech spec sheets or not, the company has the data.
They have to in order to classify an oil as a certain multi-viscosity grade. Of course, this does not apply to monograde oils although some companies run this test on those oils as well.
WHAT DO THE NUMBERS MEAN?
Most people believe that a 5w30 oil is good for cold weather use because it is a "5 weight" oil in cold temperatures and a "30 weight" oil at high temperatures. On the surface this might seem to make a certain amount of sense. Naturally, a "5 weight" oil would flow better than a "30 weight" oil. This would make it ideal for cold temperature operation.
Nevertheless, this is a profound misunderstanding of what the labeling means. The two numbers really have little to do with each other. The final number based upon the kinematic viscosity at 100 degrees C, as we discussed for monograde oils.
So, if a multi-grade oil, when heated to 100 degrees C, falls within a certain kinematic viscosity it is classified as a certain SAE grade (the last number - like the "30" in 5w30). In other words, the kinematic viscosity of a 5w30 multi-viscosity oil falls within the same range at 100 degrees C as a monograde SAE 30 weight oil does.
A multi-viscosity oil also has to meet a "High Temperature/High Shear" requirement, but I'll talk about that in a minute.
The first number (the "5" in 5w30) is only a relative number which basically indicates how easily it will allow an engine to "turn over" at low temperatures. It is NOT a viscosity reference. In other words, a 10w30 is NOT a 10 weight oil in cold temperatures and a 30 weight oil in warm temperatures.
In fact, since SAE viscosity classifications only apply to an oil at 100 degrees C, it doesn't even make sense to label it as a certain SAE viscosity at any temperature other than 100 degrees C.
Besides, if you thought about it for a second, it wouldn't make sense for a 10w30 oil to be a 10 weight oil in the cold and a 30 weight oil in warm temperatures. What liquid do you know of that gets "thicker" as its temperature increases or "thinner" as the temperature decreases?
I would venture to say you probably can't come up with one. This holds true for motor oil as well. If a 10w30 was a 30 weight oil at 100 degrees C and a 10 weight oil at cold temperatures, that would mean it "thinned out" as the temperature dropped. That just doesn't make any sense considering what we know about liquids. It just doesn't happen like that.
The fact is that a 5w30 motor oil is thicker in cold temperatures than in warm temperatures. However, a 5w30 motor oil will be thinner than a 10w30 motor oil when subjected to the same low temperature conditions - because the "W" number is lower. This is an indication of better cold weather performance.
In other words, a 5w30 flows better in cold weather than a 10w30 motor oil will. Think of the "W" as a "winter" classification instead of a "weight" classification.
Results from the Cold Crank Simulator (CCS) and Mini-Rotary Viscometer (MRV) tests are used to determine the oil's "W" grade. The better the engine "startability" of the oil at low temperature, the lower the W classification. Each W grade must meet certain "startability" requirements at a specified temperature.
For instance, a 0W grade oil must have a maximum CCS centipoise (cP) value of 3250 @ -30 degrees C as well as a maximum MRV cP of 60,000 @ -40 degrees C. A 5W grade oil must have a maximum CCS cP value of 3500 @ -25 degree C and a maximum MRV cP of 60,000 @ -30 degrees C. The lower the cP value for both specifications, the better.
Notice that the 0W grade oil is tested at a lower temperature on both tests AND must still have a lower CCS cP value than a 5W oil which is tested at a higher temperature. As a result, a 0w30 will allow your vehicle to start easier on a cold morning than a 5w30 will. Likewise, a 5w30 oil will pump easier in cold temperatures than a 10w30 oil will.
Nevertheless, at 100 degrees C, they all fall within the same kinematic viscosity range. Therefore, they are all classified as SAE 30 weight oils at 100 degrees C. In other words, after your engine has warmed up, a 0w30 and 10w30 motor oil are basically the same thickness (within a certain SAE specified range).
Of course, although this is true when the oil comes out of the bottle, we'll see in the next section that, with petroleum oils at least, the viscosity that comes out of the bottle may not necessarily be the viscosity that you find within your engine after a short period of driving.
Multi-viscosity oils provide a great deal more flexibility to protect an engine over a wider temperature range than monograde oils do. Obviously, this should be considered a good thing. However, there is a drawback to multi-viscosity oils. When manufactured from a petroleum basestock, they tend to "shear" back very easily. In fact, this was already alluded to in the previous chapter.
You see, the waxy contaminants within petroleum basestocks crystalize in cold temperatures causing them to "thicken" and become hard to pump. So, in order to allow for good flow characteristics at low temperatures, in addition to using pour point depressant additives, petroleum oils must start with a very "thin" basestock.
For instance, let's look at a 5w30 motor oil. In order to flow well enough to meet the 5W classification, a petroleum oil would start with a very thin basestock (maybe one that would be classified as an SAE 20 weight oil if heated to 100 degrees C). Then, that basestock would be combined with pour point depressant additives.
Remember from the last chapter that these pour point depressants help the basestock maintain its low viscosity even at low temperatures. They counteract the crystallization of waxy contaminants in the oil.
Thus, the oil maintains it's viscosity instead of thickening up as the temperature drops. But, in order to meet the requirements to be classified as an SAE 30 oil, something must be done to assure that this oil won't thin out to it's 20 weight basestock viscosity at 100 degrees C. The oil must be "built up" using the long-chain, high-molecular-weight polymers (called Viscosity Index Improvers) discussed in the previous chapter.
These polymers expand as temperature increases counteracting the natural thinning action of heating an oil. So instead of thinning to a 20 weight classification, the oil only thins to a 30 weight classification.
NOTE: Remember, don't let the 5W fool you. It's not a viscosity classification. It's a classification to establish that an oil will flow adequately at cold temperatures to protect your engine. The oil is still
THICKER at cold temperature than it is at hot temperatures. The oil will thin as the temperature increases. The only question is how much. VI improvers reduce this thinning action to acceptable levels so the oil can meet both the 5W requirements and the SAE 30 requirements.
Now, let me just say that, the more you think about this issue of viscosity, the more your brain is going to hurt. Just remember that petroleum basestock viscosities are prone to significant change as the basestock is heated and cooled. As it cools, it "thickens" and as it heats it "thins".
To counteract that, petroleum oil manufacturers start with a basestock "thinner" than the "30" in "5w30" (or thinner than the "40" if they're manufacturing a 10w40 oil, etc.) and add pour point depressants so the oil stays as thin as possible in cold weather. Then they add viscosity index improvers that expand with increases in heat so that the oil will not thin out too much to meet the 30 weight classification at 100 degrees C. This is how they meet the multi-grade specification.
Unfortunately, long chain polymers (VI improvers) are more unstable the longer they are. The nature of petroleum basestocks necessitates that they be "built-up" using very long chain polymers. Therefore, these long chain polymers break down fairly quickly. In turn, over a short period of time, a 5w30 petroleum oil may actually "shear back" to a 5w20 (or lower) as these polymers break down. Obviously, this can lead to a decrease in engine protection.
For this reason, to assure at least a minimum amount of protection, the SAE J300 describes another requirement that a multi-viscosity oil must meet in order to be given its multi-viscosity classification. It must maintain a certain cP level on the High Temperature/High Shear (HT/HS) test (ASTM D 4683).
This test must be performed in order to label an oil as a certain multi-grade classification because automobile manufacturers use the physical requirement standards listed in the SAE J300 in order to establish which viscosity grades should be used in which vehicles. If automotive and lubricant manufacturers are not working off the same play book, it's your engine that's at risk.
But, let's get back to the HT/HS test. If the oil shears back too much on this high temperature test, it cannot be sold as a multi-grade oil. In fact, the test results from this test are very helpful in indicating the quality of the oil. Since they're required for achieving a certain SAE viscosity grade classification, the manufacturing company has the data. Make sure you get it.
The higher the HT/HS number the better because this indicates less shearing. Petroleum oils tend to have low HT/HS numbers which barely meet the standards set by the SAE J300 specifications.
Also, because the petroleum oils are made with a light weight basestocks to begin with, they tend to burn off easily in high temperature conditions which causes deposit formation and oil consumption.
As a result of excessive oil burning and susceptibility to shearing (as well as other factors) petroleum oils must be changed frequently.
The good news is, not all multi-viscosity oils shear back so easily. Synthetic oils contain basically no waxy contamination to cause crystallization and oil thickening at cold temperatures. In addition, synthetic basestocks do not thin out very much as temperatures increase.
So, pour point depressants are unnecessary AND higher viscosity basestock fluids can be used which will still meet the "W" requirements for pumpability. In other words, it might be possible to meet 5W requirements with a synthetic basestock that would be classified as a 25 or 30 weight oil at 100 degrees C. Hence, little or no VI improver additive would need to be used to meet the 30 weight classification while still meeting 5W requirements.
The result is that very little shearing occurs within synthetic oils because they are not "propped up" with viscosity index improvers (long chain polymers). There simply is no place to shear back to. In fact, this is easy to prove by just comparing synthetic and petroleum oils of the same grade. Synthetics will generally have significantly higher HT/HS numbers. Of course, the obvious result is that your oil remains "in grade" for a much longer period of time for better engine protection and longer oil life.
Specs exlained:
P/S/B = Petroleum or Synthetic or Synthetic Blend
Noack Volatility (DIN 51581) 250°C for 1 hour, % weight loss
API = Most Current American Petroleum Institute Specification(s)
TBN = Total Base Number (ASTM D-2896)
VI = Viscosity Index (ASTM D-2270)
4 Ball = Four-Ball Wear Test (ASTM D 4172) Scar diameter, mm
PP = Pour Point (ASTM D-97)
Phos = Phosphorus ppm
FP = Flash Point (ASTM D-92)
Zinc = Zinc ppm
FRP = Fire Point (ASTM D-92)
CCS = Cold Crank Simulator Apparent Visc. @ x°C, cP (ASTM D 5293)
HT/HS = High Temperature High Shear Viscosity (ASTM D-4683)
Ash % = Sulfated ash percentage of motor oil volume (ASTM D-874)