Prius oil - dealer's choice?

The Xw-XX number isn't related to the actual viscosity, but an acceptable range for the grade. 10w-40 motor oil and 75w-90 gear oil(if I got the numbers right) have the same actual viscosity. The SAE there is trying to prevent additives going to the wrong place. A thick 0w-20 can have the same viscosity at operating temp as a thin 0w-30.

 

I have a feeling they're gonna stick with 0w16 regardless...

 

Not really. SAE J300 viscosity definitions are pretty clear. Perhaps only a boutique 0W-20 can be thicker than a 0W-30. The important viscosity is the high-temperature, high-shear (HTHS) viscosity, and for 0W-20, it is around 2.6–2.7 cP, and for xW-30, it is around 3.0–3.1 cP, which makes a big difference. There are also Euro xW-30 oils with HTHS 3.5 cP or higher, which is as high as for a 10W-40.

See the second from last page for details on SAE J300:

https://www.infineuminsight.com/media/2841/api-engine-oil-classifications-brochure2.pdf

 

Yes, SAE 0W-8 and SAE 0W-12 have been in the works for quite some time now, and Toyota has been experimenting with them. They are in SAE J300, but they are not in ILSAC yet. However, they are in JASO GLV-1.

Here is what I wrote on BITOG a while back:

0W-8 is here: JASO GLV-1 | Bob Is The Oil Guy

 

And the low shear rate kinetic viscosity at 100C(normal oil temps in the engine) listed there is in a range. A range without gap between the oil grade ratings. There is overlap of the -8, -12, -16, and -20 oils. This is just an example of how the viscosity grade numbers can't be used to infer actual properties. Seeing one claim of 0w being 'like water' was enough for me.

 

Preach!

I swear next time I hear that I'm going to serve the utterer a chilled glass of it. Rocks with a twist if I'm feeling charitable. Bottoms up!

 

If you look at the SAE J300 table in the Infineum API oil classifications I linked above, there is no overlap in kinematic viscosity at 100 ℃ (KV100) for SAE 20 and higher. Also, there is no overlap in KV100 between SAE 8 and SAE 16.

Again, the important viscosity is the HTHS viscosity.

It is true that the HTHS spec is only a minimum and there is no maximum, oils will be typically at or just above that minimum to maximize fuel economy, as a SAE 0W-16 oil is unlikely to pass the fuel-economy tests if it has a SAE 0W-20 HTHS. Some Euro categories are an exception, and they specify a higher HTHS viscosity for better protection against engine wear (such as ACEA A3/B4, C3, C4, etc. that specify a minimum HTHS viscosity of 3.5 cP regardless of KV100).

And of course, the 0W- part, which only denotes very cold-engine starting, is confusing for a few. A 0W-40 is obviously thicker than a typical 5W-30 at operating temperature. Oils with a higher xW-y spread tend to contain more plastic viscosity-index improvers and may not always be favorable though, as they can shear and reduce the viscosity or form deposits and sludge.

 

An oil with viscosity of x<y and another of y<z is close enough to say no gap for this type of discussion. I didn't claim 8 was overlapping 16, just that there was overlap among those four thinner oils.

I don't care if a viscosity measured at overheating temperatures is more important. Is there a mathematical connection between the HTHS and grade value?

 

HTHS is not only measured at high temperature (HT). It is also measured at high shear (HS). The latter is the key. KV in SAE J300 is measured at low shear (or no shear).

There is no direct connection between KV100 and HTHS, even though they do correlate.

Since HTHS is measured at 150 ℃, the viscosity index of the base oil definitely matters. Higher the viscosity index (VI), the higher the HTHS will be, as the oil will thin less with increasing temperature.

Since it is a "shear" viscosity, HTHS also depends on the viscosity-index improver (VII) content as well as how much the VII temporarily shears (not to be confused with permanent shear with VII molecules having been damaged). Shear refers to fast sliding and closely separated parts, which align the VII polymer molecules in the direction of motion and reduces their contribution to the viscosity. If there were no VII, HTHS would be the same as KV150, except multiplied by oil density to convert it into dynamic viscosity. Since there is VII in multigrade oils, HTHS will be less than KV150 (times oil density), getting smaller with increasing VII content. Multigrade oils reduce HTHS by increasing the VII content to improve fuel economy, but reduced HTHS could also increase engine wear, and more VII could lead to more engine deposits.

 

Which has nothing to do with my point about how the viscosity grade has no direct relation to the oil's properties. Which means you can't infer that 0w-8 is half as thick as 0w-16.

 

Of course it does have a direct relation to the oil's properties. A SAE 0W-8 oil has an HTHS viscosity of 1.7–1.8 cP vs. an HTHS viscosity of 2.3–2.4 cP for a SAE 0W-16 oil. (Their HTHS viscosities need to be at or just above the minimum-HTHS-viscosity specs to pass the fuel-economy tests.) That is a huge difference in viscosity.

Viscosity is the most important physical property of an oil, as it determines its lubrication behavior. As I also explained, the high-shear viscosity is what the engine actually experiences, as the oil is in a high-shear environment (fast-moving sliding parts).

Below a certain HTHS viscosity, the oil may not prevent metal-to-metal contact in the bearings, and even a few tenths of a cP can result in excess wear. That's why 0W-16 oils, which were only recently incorporated into ILSAC, are in an entirely different ILSAC category (GF-6B) vs. higher SAE viscosity grades (GF-6A). Again, that's also why 0W-8 and 0W-12 are still not allowed in ILSAC. Chances are that there will be a new ILSAC category for them. They will need to do a lot of engine testing until then to ensure that these new lower SAE viscosity grades can sufficiently protect against engine wear. Engines may need to be designed differently to accommodate these new lower viscosities as well. At the end, there will a small benefit in fuel economy, hopefully with no significant effect on engine wear.

 

Of course, water does not have zero viscosity. (It has an HTHS viscosity of 0.18 cP.) The only things that have zero viscosity are superfluids, like helium below the critical temperature of superfluidity.

 

I wouldn't be so sure. Gen 4 Prius switched from 0W-20 to 0W-16 mid-cycle. New Corolla, Crown, etc. models have 0W-8 as the recommended oil. We could see 2025 Prius switching to 0W-8. Given that Prius uses the Atkinson version of the Corolla engine, which has less torque and probably runs cooler as well, I am surprised that they haven't already switched it to 0W-8.

 

Right, you kept repeating the trivial fact that the numbers in the SAE viscosity grade did not actually correspond to viscosities, which is already known to virtually everyone here, without bringing anything useful into the discussion and educating yourself, as you didn't even know what the HTHS viscosity was and why it was the important viscosity in an engine and the kinematic viscosity did not really matter.

 

The price thing is temporary. It is being made by ExxonMobil, and it is probably cheaper to make a 0W-8 than a 0W-16, as it is thinner and has a smaller cold–hot viscosity spread. Give it a year, and it will be mainstream and cheap.

 

Virtually
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I have literally seen "0w is like water" comments, so there is plenty of ignorance of what the viscosity numbers on the bottle out there. Which can be addressed without delving into the fine details. Yet you kept going into the fine details as if the 'trivial fact' you finally acknowledge here was not so.

As for which viscosity is more important, note that Exxon doesn't include it on product spec sheets.
Mobil 1™ 5W-20 Synthetic Oil | Mobil™ Motor Oils

 

Ah, yes, ExxonMobil has not been usually including the HTHS viscosity in their product sheets lately because it considers it a proprietary spec, as knowing it can help you deduct the viscosity-index improper (VII) content and even the base-oil viscosity index. I developed such a calculator:

Estimated HTFS (base-oil + DDI-package dynamic viscosity @ 150 °C (BO DV150)) and viscosity-index improver (VII) content of selected oils

The HTHS viscosity wasn't part of SAE J300 until 1993. However, by the late 1980s, it had been clear that the kinematic viscosity did not describe the lubrication behavior of multigrade oils (SAE xW-y) as it did for the monograde oils (SAE xW or SAE y) because the multigrade oils contained viscosity-index improvers (VIIs) that temporarily (and even permanently) sheared in high-shear environments (fast-sliding parts), which decreased their contribution to the viscosity with the increasing shear rate. This is the so-called non-Newtonian behavior. Hence, the HTHS viscosity, which is measured at a shear rate of one million per second (as opposed to zero for the SAE J300 kinematic-viscosity specs) was introduced by the late 1980s. The HTHS viscosity describes the wear protection and fuel economy of multigrade oils much better than the kinematic viscosity.

High-temperature, high-shear (HTHS) oil viscosity—Measurement and relationship to engine operation—by James A. Spearot · 1989

The bottom line is that for multigrade oils, the HTHS viscosity must be used to determine their lubrication behavior in an engine. The kinematic viscosity cannot be used. In fact, you were somewhat right about the latter.