Does Pour Point change with mileage?

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Staff member
May 27, 2002
Guelph, Ontario
I was wondering if the pour point of an oil changes drastically from new oil to used oil? If a new batch of oil has a pour point of say -40F, will it end up being -30F after 3000 miles?
Considering the fact most luibricating fluids increase in viscosity as the fluid ages, yes, in general, the pour point should suffer.
Do you know by how much though? I'm just wondering if it's a small change or a much larger change? I'd hate to think that my oil in the middle of a cold February stretch, is not performing as well since it's probably going to be a couple of months and a few thousand miles old by then. I don't typically change my oil again during the winter period, especially if there is a lot of snow on the ground.
Perhaps another thing to consider folling the line of MolaKule (I don't imply that I am on his level) is that viscosity increases mainly due to the fact that the lighter molecules "boil off" making it thicker and hence higher higher point. On the other hand the shearing down of oils which is as I understand mainly the shearing of the Polymer which makes the oil thinner than it should be at operating temperatures. I would guess this action alone would not affect the pour point which is dependent on the base and is really not sheared?? Now if both conditions occur at the same time the oil could thin out at higher temperatures and the pour point still could be higher?? If that makes sense. Correct me If I'm wrong.
You know oils have come a long way in the last several years, some of it has actually been beneficial...he he! The API says that the SL grade of oil is supposed to have better (lower) volatility levels, meaning it should be losing less to boiling off/evaporation than older grades used to. I CAN say that I used to have to clean out my PCV valves every couple thousand miles with carb cleaner, but the last couple years, the valves have actually been very clean, if there was much oil vapor coming off, it would definitely show up there. I think the average molecular weight these days for oils is a pretty narrow spread compared to what it used to be, which is a good thing! HOWEVER.... Keep in mind the power of COLLIGATIVE properties.. Used oil has more particles in it than clean oil does, and in general that will raise the boiling point AND lower the freezing point (just like adding antifreeze to water).....According to THAT load of #$%^&&, you would think that it would still pump at low temps,,probably would if that was the only consideration. I have taken used oil and put it in a jar and put some new oil in another jar and put them both in the freezer, the used oil moved slower than the new oil did. I have never lived in the "GREAT WHITE NORTH", but I think that if I did, I would probably be using the synthetics that handle the extreme cold and oxidation problems better than the conventionals do if I wanted to go for long intervals, otherwise just use the conventional for 3k and change it. Here in NC I've never had a reason to go with the synthetics. Of course you can just change the oil to a winter weight at the beginning of winter too, and go back to a summer weight the next summer. Glad I live in NC!!!
I used to run 5w30 in winter here and 10w30 the rest of the time with most of my cars, but I honestly think that was unecessary, especially when our winters really aren't that cold. Besides, the lower weight number (the 5 and the 10) are viscosity equivalents at 40 celcius, and I believe that at the much colder temps the viscosities of the two oils are a lot closer. For instance, in the test I did where I stuck bottles of oil out on a well below 0F night, I noticed that the 5w30 and 10w30 Mobil 1 appeared to flow pretty much the same. So I think if you use an oil with a good pour point, there is no need to go with 5w30 over 10w30. When you compare a lot of oils pour points between their 5w30 and 10w30, often the difference is very small (for instance with Maxlife there is only a 3c difference in the PPs between those two, so I think they'd both flow the same in the extreme cold) Since Maxlife 10w30 has a PP of minus 36c, much colder than I'll ever see here, I feel it's safe to use in winter.
Originally posted by Patman: ...Besides, the lower weight number (the 5 and the 10) are viscosity equivalents at 40 celcius, and I believe that at the much colder temps the viscosities of the two oils are a lot closer...
Actually the 5w grade is determined at -30C and it has to be less than 6600cP viscosity. 10w has to have a viscosity of less than 7000 cP at -25C. It's on page 8. And it you look at the cSt of for example Mobil1 @ 40C, 0w30,5w30, and 10w30 are all very close. Which is why I don't understand why people in warm climates would even consider a 0w or 5w-30. [I dont know] They would give absolutely no startup flow advantage above freezing temperatures. However some people seem to think they do. Someone on a BMW forum uses 0w30 in Florida because he thinks it protects at startup better and that is the most important. Maybe if he lived in the artic. Nevermind that it is either going to shear back to donkey urine or oxidize and leave deposits more readily. [crushedcar] I don't see the advantage. AMSoil did a great job brainwashing with their 0w30 series 2000 "super oil" [Off Topic!] Edited to change one word. [ July 02, 2002, 11:44 PM: Message edited by: 59 Vetteman ]
How could the weight be determined at -30c when some dino oils don't even flow at that low temp though?
The SL oils handle this problem better than the older oils, as part of the test for viscosity increase used to limit the increase to 375% in 64 hours of testing. It was changed to 275% in 80 hours. But the Ford certification raises the standard to 200% in 128 hours.
Correct me if I'm wrong, but does this mean that the current SL rated 10w30 oils behave very similar to what the SJ rated 5w30 oils would? So in essence the need for 5w30 oils is even less?
Originally posted by Patman: How could the weight be determined at -30c when some dino oils don't even flow at that low temp though?
Well I'm not quite sure. But nowadays I think that all oil of the respecitive grade will flow at those temp. The CCS test doesn't measure flow anyway so it wouldn't matter. Bob's site has a short explanation: Maybe someone else can quantify the centipoise numbers. I really don't know what it means, is 7000 thick/thin/average? I would guess that they would at least set the ratings where a real engine could be turned over at that temp. And as you can see each grade is only 5C difference. So there really isn't that much difference. E.g., a GM owners manuals say 10w30 is ok to 0F, a 15w should be good to approx 10F. So it really isn't THAT THICK. Because I have seen people balk at someone putting a 15wX oil in a new car...but it really isn't that big of a deal. I'd even feel fine using it in the winter if it wasn't extremely cold. In fact I did use Mobil1 15w50 in my car in the winter with good results.
Patman, The long term viscosity increase for dino oils is caused by higher volatility compounds evaporating and leaving behined the higher viscosity compounds. In addition, VII's and other additives shear back and add to the thicker oil. The only paper I could find on this topic was by Chamberlin (of Lubrizol) comparing PAO/esters with mineral oils. The viscosity increase at -18C (over a 40 kilometer test): Mineral went from a 10 W to a 22W. Synthetic went from a 10W to a 13W. MolaKule
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