gurus...need help with viscosity

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Jul 30, 2002
richmond, va, usa
I was researching some of the oils that I have used, comparing them to some that I am thinking of going with and found the following:

oil Visc at 40C Visc at 100C

Valvoline 10w30 70.8 10.5
Delo 15w40 116 15.6
Rotella T 15w40 100 14.4
Rotella Syn 5w40 88.8 14.6

I usually use 10w30 in winter, 15w40 in summer...but was thinking of using the Rotella Synthetic this winter. To my untrained eye though, the Rotella 5w40 looks thicker at 40C than the Valvoline 10w30.

Those in the know, please help me understand. I tried looking at the visc chart, but that didn't help.


[ August 30, 2002, 10:30 AM: Message edited by: matt cook ]
5W is not based on the vis at 40C which is 104F.

W stands for Winter and the 5W is an indication of how it will behave at cold start during winter.

It is based on it's pumpability at a certain temp.

The viscosity chart is for single grade oils with a VI of 96. It's an aid, but not really aplicable to multivis oils.

Edit: Oops, sorry... not a guru.

[ August 30, 2002, 10:52 AM: Message edited by: VaderSS ]
The xxW is the viscosity for cold temps, and will indicate through the chart Bob has somewhere here the pumpability at different cold temperatures.
the "30" or "40" represents the viscosity at 100 C. a SAE 40 has to be from 12.5 > 16.3 cSt. A 30 will be "thinner". Within the category there are minor differences that are not significant. More important is where it will be after 3000 or 4000 miles, and a Group II or Synthetic will probably be closer to the original value than a Group I product. In the many analysis I have of Delo 15w40 I have not seen it either shear down or increase in visc. I haven't tested any of the others you mention.
I still don't fully understand how the first number in the viscosity rating works though.

I can't see how a cheap 5w30 with a -40F pour point can possibly be a better oil to use
in the winter than a good 10w30 synthetic that has a -60F pour point. But yet supposedly
if the first number is a 5, it's better than an oil with the first number of 10. I always used to
believe that the first number meant that it was the same as that viscosity oil on startup, in
other words, a 5w30 oil meant that it would behave exactly as a 5wt oil when it was cold, or
have the same viscosity in other words. But now I find out that this first number is just a
number, and not a viscosity at all. So in other words I'm more confused than ever. I'm further
confused when I read that Mobil 1's 15w50 has the same pour point (-49F) as it's 10w30. Are
we to then believe both will work the same in cold weather? And for many years the 10w30
and 5w30 Mobil 1 shared the same -65F pour point. If two oils have the same pour point, how
can one possibly be better than the other in winter? Help, I look like these guys-
There is a specified viscosity for the oil to achieve in the "cold cranking simulator." If the oil achieves that viscosity at -35C, it's called 0W. If the temp has to be -30C to reach the specified viscosity, the oil is rated as a 5W. -25C for 10W, -20C for 15W, -15C for 20W, and -10C for 25W. Note that the lighter rated vis oil is also a lighter specified vis at the testing temperature. See page 8 of this link:


[ September 01, 2002, 01:44 AM: Message edited by: Ken ]
Ken I had read that document before and it confused me then too. If I am reading it correctly, an oil that is designated as a 10w30 right now under the SL formulation, could have qualified as a 5w30 under the SJ formulation?

As Ken mentioned, the first number, 0w/5w, etc is determined by the viscosity of the oil in the cold crank simulator (CCS) test. For each 5pt difference, the test is done at a temp that is 5C(9F) colder, so going down from a 10wt to a 5wt gives you an advantage of 9F in terms of cold flow properties at mild temps. However, this tests tells you nothing about how the oil is going to work when you get down BELOW the CCS temp. So a 5w-30 petroleum oil will start to thicken rapidly below -25C and have a higher pour point than a 10w-30 synthetic.

For extremely cold weather, the more relevant number is the borderline pumping temp, or BPT. This defines the practical low temp limit of the lubricant - when you get down to the pour point, the oil is going to be too thick to pump out of the crankcase.

The SAE "J300" specifications for low temp viscosity have changed (a year ago last July, actually). The CCS temps for each SAE grade dropped by 5C, but the max allowable viscosity increased - in most cases from 3500 centipoise to 6500 centipoise. So the old and new viscosities of different oils aren't directly comparable.

You are approximately right. They lowered the temp to measure, and gave back a little in the maximum visc. So a 5w30 SJ and a 5w30 SL are not necesarilly the same. (but could be)
This change lets the synthetics and better flowing oils show better as they will always have better pour points and pump better.
But note the word "Maximum". You can (and could) make a 0w30 and call it a 5w30. You can label a 10w40 as a 15w40. If a plant wants, it can have one batch of oil in two different bottles. I doubt that you will see this with the big companies, but for small ones it is done.
I'm not worried at all about running the 10w30 Shaeffers in the winter here, as it's borderline pumping temperature is -28 to -30F, and it will never ever get anywhere near that cold in this area. It rarely goes below 0F actually. By the way, the Schaeffers is going in my car in a couple of hours! I'm just getting ready to head out the door to run some errands, and fully heat up the oil before taking my Maxlife sample. I know it sounds strange but I've never looked forward to an oil change as much as this one here!

[ September 01, 2002, 09:58 AM: Message edited by: Patman ]
Originally posted by TooSlick:

For extremely cold weather, the more relevant number is the borderline pumping temp, or BPT. This defines the practical low temp limit of the lubricant - when you get down to the pour point, the oil is going to be too thick to pump out of the crankcase.

This is the number I look for in an oil, the BPT, as it is much more relevant for northern Minnesota weather.
I moved up here in November 1987 from northern Virginia but I winterized my then Audi 4000 in Virginia with 10W40 (always used 20W50 there).
When January came and it went down to -40 below(and I thought I knew what winter was from college days in Ohio, huh) that is what started me on this whole oil quest anyway, and it then became an addiction.
It is a good thing the Audi did not start that morning. The oil stuck on the dip stick back to the engine like syrup in those old King Syrup cans.
I have learned much since then but still much to learn.


For extremely cold weather, the more relevant number is the borderline pumping temp, or BPT. This defines the practical low temp limit of the lubricant - when you get down to the pour point, the oil is going to be too thick to pump out of the crankcase.

I agree. Just because it will pour, don't mean it will pump.
Is it true that to find out the borderline pumping temperature (if it's not given in the specs) that you simply take away 20F from the pour point? In other words if an oil says it's pour point is -50F, then it's BPT is -30F? Or is it possible for two oils to have the same pour point but have two different BPTs?
TooSlick, I have read your posts with great interest,you seem very knowledgable. Would you mind taking a look at my question I had?

[ September 02, 2002, 09:01 PM: Message edited by: dragboat ]

I can't open this link ...just e-mail me the question and I'll be glad to provide you with an answer.

I just swiped it of the original post that either got missed by the ones that would know,or no one knows?

It has been a number of years back"1998" but I opened a 5 quart container of the Superflow oil in un heated shop cold storage for a oil change and it was jelled! What is your opinion on how this happens?

Could this possibly happen in the engine during the winter months with out knowing? Eitherway,does not seem good!
Base stock problem?End off effective shelf life? Was using it in a beater,5 quarts @ 3.99 for 5 quart jug at Autozone

[ September 02, 2002, 09:09 PM: Message edited by: dragboat ]
back to the original question...

If I decided to run Rotella Synthetic 5w40 in the winter and dino Rotella 15w40 in the spring/summer/fall would there be any issues - it is okay to switch between syn and conventional, but it would be better if I stayed within a brand, correct?

The BPT is generally 15F-20F higher than the pour point. Pour point is actually something of a misnomer - it's actually 5F above the temp where the oil is essentially solid.


Yes, if you stay within a brand, then the additive chemistry should be similiar. I'd probably just use the 5w-40 Rotella year round if it were my engine.

I'd be willing to bet that the additive chemistry between the dino Rotella and synthetic version is different enough that it could cause a slight problem. Looking at virgin analysis results between a dino oil and a synthetic from the same company often shows quite a bit of difference. Some synthetics will have a lot of magnesium for instance, while the dino version has little or none. Also, between a 15w40 to a lower weight version of an oil, this also can show a big difference, such as how Pennzoil 15w40 has 153ppm of moly, while their 10w30 and 5w30 have none.

So if he is going to switch back and forth between these oils from winter to summer, then long drain intervals would not be recommended, at least not on the very first interval after the switch to the different oil.
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