10W-40 wimpy, 15W-40 tough

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The reputation is that 10W-40 oils break down quickly. Some think the 30 point viscosity spread requires greater VIs, which shear quickly, losing viscosity. 15W-40 oils get high praise for resistance to shearing and breakdown. They have a 25 point viscosity spread, 5 less than 10W-40. Several questions: Is there really that much difference between these oils? Does the additional VIs required to get 5 more points make the 10W-40 dramatically more susceptable to shearing? Is the additive package that much more robust in 15W-40? Is the main difference between 10W-40 and 15W-50 greater amounts of ZDDP in 15W-50? Is the bad reputation of 10W-40 based on older oils, and are modern 10W-40's very good like most oils. Why does 10W-40 have such a bad reputation?
 
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15W-40 HDEO's are much 'tougher' than 10W-40 PCMO's. 15W-40's use a much heavier base oil, so they ofter have a HT/HS of 3.5 or better, as do 'A3' euro oils. PCMO 10W-40's CAN have a high HT/HS value, but they are only required to have the same as a 30-weight, or 2.9. (Not 100% sure I have that right, I read something like that by one of our more knowledgable members once...) 15W-40 do usually have a much stronger add-pack in them, to take the abuse of diesel service - they have higher Calcium/Zinc/Phiosphorus amounts, and a higher TBN value. In general, most PCMO 10W-40's are just 'SM' rated, and have the same add-pack as 5W-30/10W-30's. With that said, there is nothing wrong at all with modern 10W-40's. They are made with the same quality basestocks that the 30-weights are, and a modern 10W-40 and 5W-30 often have the same VI. The bad rep 10W-40's have is mainly from the 1970's, when group I basestocks were used, and a lot more viscosity index improvers had to be used to make the spread. These VII's were also poorer quality, and reacted badly to heat and broke down, causing sludge.
 
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Yup. a 10W40 is only required to meet the HT/HS of a xW30. It could be built tough, but most PCMOs are not built that way. I thought I read somewhere they would be changing that so 10W40 has to meet the rest of the 40 grade soon, but Im not quite sure where I read it, or if its true.
 
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 Originally Posted By: addyguy
15W-40 HDEO's are much 'tougher' than 10W-40 PCMO's. 15W-40's use a much heavier base oil, so they ofter have a HT/HS of 3.5 or better, as do 'A3' euro oils. PCMO 10W-40's CAN have a high HT/HS value, but they are only required to have the same as a 30-weight, or 2.9. (Not 100% sure I have that right, I read something like that by one of our more knowledgable members once...)
That is correct. Here is a link to SAE J-300. http://www.finalube.com/reference_material/SAE_Viscosity_Grades_For_Engine_Oils.htm I think the lower HTHS is how M1 0W-40 can say "Energy Conserving" in the "donut".
 
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