How can synth oils use same viscosity grades as dino yet be better?

[kanling]

One of the major advantages of synthetic oil is that it maintains its viscosity better at lower temperatures. To me, that means it doesn't thicken up as much as a dino oil and is better for cold morning starts.

But, if two oils (dino and synth) are rated as 5w30 shouldn't they have the same viscosity at operating temp as well as whatever temp the "5W" is spec'd at?

Where does the viscosity advantage of synthetic show up?

Another point: How can there be 0W-30, 5w30, 10w30 grades of synthetic if its viscosity holds up so well at low temps? What are they doing to change the viscosity to meet these different grades?

Something doesn't make sense here. What am I missing?

 

[Pablo]

Think specification ranges and classifications that are minimums/maximums.....

 

[Ray H]

Also think in terms of a finished oil that needs little or no VIIs to meet its rated viscosity range.

 

[y_p_w]

Also acceptable protection/higher viscosity at temperatures well beyond a "normal" operating range. Think of a hot day in a car without an oil cooler. I hear the oil temps could go as high as 280 deg F in some cases.

 

[mechtech]

Right - it's the performance when things are really hot or cold.
Like 25F below 0.
Or 260F.
For most normal temps, the major difference is the longer drain intervals of the synthetics.

 

[TallPaul]

A conventional 5w30 will have a lot of viscosity index improvers (VIIs). A synthetic will have much less VIIs and in some cases a high end synthetic (Redline) will have no VIIs.

That may explain why many 40 weight synthetics are 5w40, whereas conventionals are 10w40. And I bet the synthetic 5w40 has no more VII than a conventional 10w40 (and probably less).

Also, the molecule size distribution is much tighter in a synthetic and that may lead to better or freer flow in the synthetic as it should have less turbulence.

A synthetic is purer. Conventional has residuals from the crude oil, whereas synthetic is constructed separate from the original crude oil, except Group III which is highly purified through additional refining processes.

 

[mdocod]

since the POA stocks contain oil molecule strand lengths that are all almost exactly the same- the oils flow easier than similar weight dino oils. Better flow and penetration results in cleaner engines. Since a pure synthetic is not "refined" from crude- there are no impurities to begin with, so less potencial for contamination- longer OCIs become possible. Better flow also means more oil goes through the filter and less through the bypass. Less strain on the oil pump, slightly less overall resistance in the engine- sometimes resulting in slightly better fuel economy.

 

[kanling]

So, what I'm getting here is that unless you are operating at very high or very low temps, a 5w30 synthetic is no better than a 5w30 dino from a viscosity standpoint.

The synthetic has advantages of fewer VIIs, longer drain intervals, etc.

But the viscosity will be the similar at summer operating temps where you are NOT driving up mountains and pulling a trailer. And also the viscosity will be similar when you are not living where the winter temp goes down below say 0 F.

 

[Blue99]

Here's the Infineum SAE J300 Viscosity Chart .

As Pablo commented, it's best to think ranges for the xW cold temp ratings.

A 5W oil must be max 6600 centipoise @ -30C to meet the 5W grade..

5w30 Grp II dino oils generally run 5300-5800 cP @ -30C.

M1 5w30 is 3600 cP @ -30C.

PAO's are generally recognized as having excellent cold flow properties, while the cold flow performance of a dino will generally degrade over the OCI interval as the pour point additives break down or burn off.

 

[kanling]

So what I've learned here is that synthetics are likely to be less thick at low temps than dinos in the same XW-XX viscosity grade.

But, making a blanket statement that "all synthetics stay thinnner in cold weather" is NOT accurate- a 10w30 synthetic is very likely thicker than a 5w30 dino at -30 C.

Then again, I never operate at -30C... so who knows how they would compare at temps I do operate under... I guess I need curves... I doubt it is very linear judging by the 1.7 factor for 5 degrees!

 

[kanling]

Good answer, Blue99. By the chart, then, it appears that any 0W oil is also a 5W oil.

So, if a synthetic manufacturer is selling a 0W and a 5W as different products it would logically mean that the 5W bottle doesn't meet the more stingent 0W specs.

It seems like "logic" is not prevailing in the marketing department. Your M1 5w30 with 3600 cP @ -30C is clearly MUCH better than 0w30 specs! So why does Mobil sell a separate M1 0w30 and a M1 5w30?

They could save retailer shelf space with a single "0W/5w30".

Are they doing it just to pacify confused consumers whose manuals say "use 5w30"? Are M1 0W and 5W identical but just in differently labeled bottles?

 

[MolaKule]

quote:

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Are M1 0W and 5W identical but just in differently labeled bottles?

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No they are not identical. Most likely formulated with slightly different starting-viscosity base oils and differing amounts of VII.

Synth oils flow better and are more stable because their molecular structure consists of well structured molecules of consistent size and molecular weight.

 

[Blue99]

The key is that each xW grade, on the J300 chart, is at a lower increment of -5c degrees.

So a 5W is rated at -30C, a 0W at -35C.

At cold temps, a synthetic oil will increase by a general factor of 1.7 for every 5C degrees.

So the 3600 cP of M1 5w30 @ -30C becomes 3600 x 1.7 = 6120 cP @ -35C.

On paper, the 6120 cP figure slides under the 0W rating of 6200 cP @ -35C.

But I seriously doubt M1 5w30 actually meets the 0W rating as we are just doing "paper" calculations and the actual results can swing wildly when oil is turning to syrup at these cold temps.