Do you think that's "thick" oil? Think again!

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Dec 29, 2010
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Recently there has been concerns regarding premature timing-chair wear, which resulted in people using thicker oils. Many people think that when they use a 0W-40, they are putting in a thick oil because it's "40-grade" when it's at normal operating temperature. It turns out that it's not that simple. This Nissan study says that timing-chair wear has to do with the base-oil viscosity, not the finished-oil viscosity, which has more to do with the amount of the viscosity-index improver (VII) polymer blended in than the base-oil viscosity. It turns out that for boundary lubrication (metal-to-metal contact), such as in the valvetrain or timing chain, finished viscosity has no effect as the VII molecules are squeezed out and only a microscopic layer of oil is present in addition to the antiwear/extreme pressure/friction modifier (AW/EP/FM) compounds. Nissan study on wear in timing chains This brings the question that what's the actual viscosity of the base oil rather than the VII-blended oil, which is an indicator of the oil's ability in protecting boundary- and mixed-lubrication regions (metal-to-metal or partial metal-to-metal contact). Finished, VII-blended viscosity is only an indicator of safeguard against wear in hydrodynamic region, such as the bearings. The clue on base-oil viscosity comes from the ExxonMobil guide on synthetic-oil blending. Here are the two rules: (1) As a general rule, x in xW-y is an excellent indicator of base-oil viscosity. A 5W-20 base oil is a lot thicker than a 0W-20 base oil and a 10W-30 base oil is a lot thicker than a 5W-30 base oil. So, if you really need a thick oil, such as in older engines, in TGDI engines, or any engine where timing-chain or valvetrain wear is a concern, stay of from any 0W-xx or even a 5W-xx oil. (2) Within the same xW-y group where x is a constant but y varies, such as [0W-20, 0W-30, 0W-40] or [5W-30, 5W-40}, the oil with the smallest spread -- smallest difference between y and x -- has likely the thickest base oil and the oil with the largest spread has likely the thinnest base oil. So, 0W-20 is likely to have a thicker base oil than 0W-40! In fact in my UOA comparison of 0W-20 and 0W-40, I did find out that 0W-20 produced less valvetrain wear (less iron [Fe]) than 0W-40! To summarize the rules for getting a really thick oil: Find the oil with the largest cold number x and the smallest spread between the hot and cold numbers y - x in the SAE viscosity grade xW-y. It's more crucial for the x to be large than the y - x to be small. A 0W-40 is an awful choice for valvetrain and timing-chain wear protection, as it has the smallest x and largest y - x. 15W-40 would be an excellent choice for a thick oil, because it has a very high cold-viscosity number and a relatively small spread between hot and cold viscosities, which indicates that it actually does have a thick base oil instead of having been thickened by the VII. Here are the examples given by ExxonMobil. Base-stock viscosities are implied in the names of the base stocks. For example 4 means 4 cSt and 45 means 4.5 cSt. These are the base-oil viscosities calculated from the base stocks and their percentages in the table (4.1 cSt, 5.8 cSt, 8.0 cSt, and 4.3 cSt base stocks, respectively):
Code:
SAE grade  Base-oil viscosity at 100 C (KV100 for the base oil)

 0W-20     5.43 cSt
 0W-30     5.16 cSt
 0W-40     5.00 cSt
 5W-30     6.80 cSt
 5W-40     6.56 cSt
 5W-50     6.67 cSt
10W-60     7.90 cSt
So, 0W-40 is the thinnest oil in terms of the base-oil viscosity! A final note: While x in xW-y is always a good indicator of the actual base-oil viscosity, there are variations in how much the y - x spread affects the base-oil viscosity. While ExxonMobil guide indicates the conclusions here, there are other formulations possible. You are always safe with a large x if you want a thick base oil. However, you will usually have an additional assurance on base-oil thickness if you also choose a small y - x.
 
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This is a interesting "study" but at what point can we accept it as truth or proof?
 
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Yep, straight 30W Oil often delivers outstanding used oil analysis on this form. Modern straight-grade oils often exhibit much better cold flow characteristics than their compadres from decades ago. When you look at the numbers, some straight 30W oils are ACTUALLY 15W-30 or 20W-30 oil, Even though it is labeled as a straight 30 anyway.
 
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Really a great point Linctex. The current 30 grade oils really are much better than past ones from years ago. I bet you are right that they really are 15w30 at the end of the day.
 
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Mar 10, 2015
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Thanks for posting. Counter intuitive but makes a lot of sense after thinking about it. Also thought it was interesting the study confirmed the anti-wear properties of MoDTC which I think this is the first time I seen anything other than just speculation regarding its anti wear properties.
 
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Yes, that's what I'm talkin' about. Nice find Gokh's It's always reasonable to assume that a benefit in one parameter might come at a cost in another parameter, even if one doesn't know what the catch is yet. Chasing inappropriately low W ratings on high-VI oils, this study would suggest, may unnecessarily increase actual component wear. Love those straight blends thumbsup
 

JAG

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Oct 23, 2005
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Thanks Gokhan. As you know, the “Harman Index” is a great way to quantify how close to Newtonian oils are...an indication of how much VII is used. https://bobistheoilguy.com/forums/ubbthreads.php/topics/3630984/Harman_Index_for_0W20s.... https://www.bobistheoilguy.com/forums/ubbthreads.php/topics/4406955/1 The downside is that it requires density, which some data sheets don’t provide. If it doesn’t, another good metric is ratio of HTHS to KV100. Multiply by 10 if you prefer numbers over 1 for readability reasons. It is not as good of a metric because oils of very low or high density get skewed values because kinematic viscosity is itself skewed by density. Dynamic viscosity is the “real” viscosity at low shear rates, while kinematic viscosity is not.
 
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Mar 31, 2018
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The problem in a 0w40 arrises only when the VII shears up, after a kindda long run. Whyle new fresh the 0w40 has a healthy 3.6 cst hths. The Nissan test is for a long OCI? No. It comes from start already with 0.4% of solid abrasive contaminants, too. A thicker base will give more cushion for the particles, after a temporary or permanent shearing. Keep the insolubles low and the chain will run for a long time. More than with a thicker base.
 
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Apr 4, 2016
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HTHS is a good indicator of base oil viscosity. For a given “finished” viscosity, more VII in an oil will result in a lower HTHS. I think I’ve read on here that Redline 5W-20 and 10W-30 have no VII.
 
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