HTHS = High Temperature / High Shear It is a more accurate measure of the viscosity of the oil in the engine bearings in actual use.
For a good read on it.....use the search.
I state that because I'm not as up on it as folks that have written about it, so you can get a much better description from them.
In short, there are other, VERY important specifications for motor oil than the simple Xw-XX (0W-20 etc).
2 motor oils can have the same numbers......such as 5w30 on the bottle....but the HTHS can be different between the 2.
The SAE specification lists a MINIMUM......of 2.9 for a 5w30, but there is not a stated maximum.
To find the HTHS specification for a particular oil, you normally need to go online and find a specification sheet for that particular oil on the manufacturer's website.
It is NOT normal for HTHS to be listed on the bottle.
As mentioned above, another specification is the VI = Viscosity Index = a measure of how much a motor oil thickens as temperature goes DOWN.
The LESS an oil thickens as it cools the BETTER.
This means, the higher the VI number, the better.
But then comes the question.....what do they need to do to get the high VI?......What is the quality of the Viscosity Index Improvers that they add?
Again, VI is not normally found on the bottle......but on the specification sheet on the manufacturer's website.
5w30.......
The 5W is a viscosity measurement taken at 40 degrees C.....most engine starts from cold are well below that.
So between 2 5w30 oils.....1 may have a higher Viscosity Index and be less thick (better) at 20 degrees C (near room temp) and even better at 0 degrees C (freezing).
The 30 is a viscosity measurement taken at 100 degrees C, close to the operating temperature, but a different type of measurement than the HTHS measurement, which is taken at 150 Degrees C.
Then it comes down to how well a oil STAYS within the various specifications over time, use and the various conditions to which it is subjected.
In short....it is common for folks here to get hung up on 1 specification...or 1 ingredient in the oil.....I've done it myself.
But I want to stress that it is the whole package......many different specifications that are important in determining the best motor oil for YOUR particular application.
Also, some manufacturers list more information than others on their website.....
The question here is....if a person wants a particular HTHS and they have 2 oils of the same basic chemistry (Such as Mobil 1 0W-20 and Mobil 1 0W-40), one has a lower HTHS than they are wanting and the other has a higher HTHS than they want, how do they mix the 2 to get the HTHS that they are seeking?
3*3.1+2.7)/4 = 3.0 (Along the lines of Mobil 1 Advanced Fuel Economy 0w30)
