Very well said ... There is more to oil than just Centistokes and HTHS... such as low Noack volatility, high TBN, low pour point and low wear scar to name some. I understand that some specs on PDS can be misread if you are not well informed. For example sulfated ash has to be taken into consideration with ZDDP etc if I am not wrong. Even then, reading PDS can be tricky.. For example Amsoil 0W20 reads a wear scar of 3.5 diameter which puts it in the league of amsoil 0w30. But upon closer inspection, you can see that test was ASTM D-4172B was done for 0w20 and ASTM D-4172 was performed on 0w30. Difference is that 0w30 gave the same wear scar at double the temps and higher RPM. Now, keep in mind that 5w30 and 10w30 didnt do any better than 0w20 in this regard. This is surprising because you'd expect 10w30 to do better than 0w30 in wear test. Reason could be that higher the spread, more PAO goes into base stock. This is all my conjecture at this point but thinner the spread, then perhaps adding heavy viscosity modifiers and pourpoint depressants can do the trick and relatively less PAO is needed. But props go to 10w30 in the Noack department as it beats out easily all the thinner oils of its breed. Here are the specs from amsoil site:
http://www.amsoil.com/storefront/asm.aspx
http://www.amsoil.com/storefront/tso.aspx
http://www.amsoil.com/storefront/asl.aspx
http://www.amsoil.com/storefront/atm.aspx
http://www.amsoil.com/storefront/asm.aspx
http://www.amsoil.com/storefront/tso.aspx
http://www.amsoil.com/storefront/asl.aspx
http://www.amsoil.com/storefront/atm.aspx
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Base on those fact, which one is more important? Visc @100degC or HTHS number? Or maybe other point? Thanks for reply..
With these three data points I don't know if it is as simple as which is most important.
In some ways, I wish I had added cold cranking, NOACK Volatility, and Viscosity Index as well. And even then there is far more to picking the right oil.
I think our chart just allows some quick reasearch as to whether a given oil is thinner than, typical of, or thicker than other oils in the same grade.
This might allow us to pick an 'thinner' oil for better mileage, or a 'thicker' oil for more protection in a modified engine or service use application, or to reduce oil consumption slightly.
The HTHS number should also be some help in determining if an oil will be better in a modified engine or service use application. Or, whether an oil might meet or exceed one aspect of a factory approval.
I drive hard, so I still think that a thicker oil at 100 C and a higher HTHS number means improved protection.
If I did lots of cold starts, and drove easy...then I might think that cold cranking was most important.
Other forum members might weigh in with their insights, and reveal what I have overlooked or mistated.