Mixing Viscosities.......

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nick778

Some folks like to i.e., blend some Mobil 1 15w-50 with the 10w-30 to increase the overall viscosity since the 10w-30 is on the lighter side of a 30w. Thinking about this, wouldn't you not end up with a i.e., a xx-40w but two distinct viscosities floating around in your oil system and not an average of the two? If so, I don't see where this is beneficial or even the desired result as wanting a higher 30 wt isn't the same as wanting *any* 50 weight. Maybe I'm thinking about this all wrong. Does anyone know? Your thoughts from a theoritical perspective?

Nick, it's how you'd guess. A 50/50 blend of 10W30 and 15W50 results in 12.5W40. But avoid the SAE grade and do the calculations based on actual specs:
code:
```Mobil1      10W30    15W50
cSt@100c      9.8     17.4 = 13.6 (40W is 12.5 to 16.3)```

I have doubts there's an accurate method to calculate the winter rating since they're at different temperatures. 12.5W sounds good. David

I asked this question to MoleKule in a PM a few weeks back and he mentioned to me that it's not always an exact calculation since the mixing of the two different viscosities, with two different amounts of VII, will alter things slightly. So a 50/50 mix of two oils, one at 10cst and one at 14cst, won't necessarily produce a final product of 12cst. I'm going to be mixing 10w30 and 15w40 Schaeffer Supreme this spring, but will do a quick sample one week after putting in the mixture, and will see both the viscosity at 40c and 100c.

Even when you compare the viscosity of a given wt of oil among different mfgrs there is a range of wts that would be seen for a particular wt, in other words company A 10w/30 is higher or lower at a given temp than company B 10/w30 would be. If you mix 2 different viscosities and just figure a mathematical average it will be a close approximation to the actual values, physical diagnosis would verify the exact value at given temps. Half 10w/30 and half 20w/50 would be very close to a 15w/40 in other words...most 15w/40 oils are diesel rated oil though that contains a higher detergent level which would be an added factor, but the wt value would be about the same.

I think what's being asked is...if one mixes equal parts of 15-50 and 5-30, do you get a 10-40 or just a mix of the 2?? ie. at extreme cold, only the 5-30 molecules flow and at high temps., they also burn off so there's no real advantage to mixing...other than slowing consumption..perhaps? Is this correct?

i got this from the million mile book...when you mix oils you add the wt.s then devide....i.e. 10w-30 and 15w-50 4 qt's of 10w 30 1 qt 15w-50 so 10x4=40 plus 15=55 divided by 5=11W 30x4=120 plus 50=170 divided by 5=34 so your new oil wt. is 11W-34

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Originally posted by boxcartommie22: so 10x4=40 plus 15=55 divided by 5=11W 30x4=120 plus 50=170 divided by 5=34 so your new oil wt. is 11W-34
What ou are doing is doina a weighted average. And I guess I believe Molakule when he says it won't be exact. Dr. T - I'm guessing that the solution will assume the physical properties of the resulting solution instead of continuing to operate on the microscopic mixture. In a true solution this is what occurs. (I believe-I could be wrong though) My assumption is though that a true solution is formed. [ February 04, 2003, 09:24 AM: Message edited by: Al ]

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Originally posted by boxcartommie22: i got this from the million mile book...when you mix oils you add the wt.s then devide....i.e. 10w-30 and 15w-50 4 qt's of 10w 30 1 qt 15w-50 so 10x4=40 plus 15=55 divided by 5=11W 30x4=120 plus 50=170 divided by 5=34 so your new oil wt. is 11W-34
It doesn't work that way at all. What if the 10w30 oil you use is a very thin 30wt like 9.5cst? So it's almost 20wt. Or if the 10w30 you use is 12.0 cst, almost a 40wt? Then the calculations are off. The only way to truly determine it's viscosity is to mix it and then do oil analysis after running it in the engine a few days, and see what the final viscosity at 100c ends up becoming. I'm also lucky that the oil lab I uses takes readings at 40c also, so I can see approximately how it's lower temp properties are also. (although I'd need a true cold cranking test to see it's winter behavior, but then again I'm only going to mix oils in the warm weather anyways)

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Originally posted by Dr. T: I think what's being asked is...if one mixes equal parts of 15-50 and 5-30, do you get a 10-40 or just a mix of the 2?? ie. at extreme cold, only the 5-30 molecules flow and at high temps., they also burn off so there's no real advantage to mixing...other than slowing consumption..perhaps? Is this correct?
Yes, this is what I am questioning. Assume a 30w is a baseball size molecule and the 50w is a basketball. when you mix the two weights, do you end up with a mix of softballs and basketballs or one ball bigger than a softball but smaller than the basketball?

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Originally posted by nick778:
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Originally posted by Dr. T: I think what's being asked is...if one mixes equal parts of 15-50 and 5-30, do you get a 10-40 or just a mix of the 2?? ie. at extreme cold, only the 5-30 molecules flow and at high temps., they also burn off so there's no real advantage to mixing...other than slowing consumption..perhaps? Is this correct?
Yes, this is what I am questioning. Assume a 30w is a baseball size molecule and the 50w is a basketball. when you mix the two weights, do you end up with a mix of softballs and basketballs or one ball bigger than a softball but smaller than the basketball?

I'd say you wind up with a mix of baseballs and basketballs, but it would still flow better than all basketballs. Remember, a synthetic blend that is a mixture of PAO and conventional oil flows better than the conventional oil alone.

If this is indeed the case (you end up with a mix of baseballs and basketballs) I do not see the benefit of mixing i.e., Mobil 1 5w-30 and Mobil 1 15w-50. For example, my car specs either a 5w-30w or 10w-30 oil. I wouldn't want *any* 50w basketballs in there. What I want is a larger baseball or a somewhat higher 30w viscosity for high temp protection. What am I missing here?

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Originally posted by nick778: If this is indeed the case (you end up with a mix of baseballs and basketballs) I do not see the benefit of mixing i.e., Mobil 1 5w-30 and Mobil 1 15w-50. For example, my car specs either a 5w-30w or 10w-30 oil. I wouldn't want *any* 50w basketballs in there. What I want is a larger baseball or a somewhat higher 30w viscosity for high temp protection. What am I missing here?
What you may be missing is that the 5w30 base oil blend may itself be a mix of golfballs, baseballs, and softballs, and this mix gives the 5w low temp rating. The base oil blend for the 15w50 may be a mix of softballs and basketballs, and this mix gives the 15w low temp rating. So, if you mix the two, you wind up with a mix of golfballs, baseballs, softballs, and basketballs, which gives a low temp spec somewhere in between 5w and 15w. I think MoleKule would confirm that this metaphor is a more accurate way of looking at the base oil composition than just saying that 5wXX has baseball size base oil molecules and 15wXX has basketball size ones.

"I thought one of the major benefits of synthetic over conventional is uniformity of molecule size. So by mixing different weight synthetics, one of the major benefits are diminished." Baseballs, basketballs, golf balls.... interesting analogies! Seriously, I applaud all of you for your critical thinking skills. In addition to molecular size, the molecular "structure" and "type" (the geometric arrangement of atoms) determines many of the viscosity, oxidative, shear, volatility, and other characteristics of the oil. There is a technical paper which shows how VII's and other additives affect the final viscosity of the oil, and that the simple mixing of oils of different viscosites doesn't necessarily result in an oil of weighted or median values wrt to the orignal base oils. Will try and dig it out if you like and report back, since I don't recall the specifics. [ February 04, 2003, 04:01 PM: Message edited by: MolaKule ]

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Originally posted by MolaKule: "I thought one of the major benefits of synthetic over conventional is uniformity of molecule size. So by mixing different weight synthetics, one of the major benefits are diminished." Baseballs, basketballs, golf balls.... interesting analogies! Seriously, I applaud all of you for your critical thinking skills. In addition to molecular size, the molecular "structure" and "type" (the geometric arrangement of atoms) determines many of the viscosity, oxidative, shear, volatility, and other characteristics of the oil. There is a technical paper which shows how VII's and other additives affect the final viscosity of the oil, and that the simple mixing of oils of different viscosites doesn't necessarily result in an oil of weighted or median values wrt to the orignal base oils. Will try and dig it out if you like and report back, since I don't recall the specifics.
So, does this mean that, all things being equal, that if someone mixes baseballs and basketballs, you end up with a melon size molecules or a bunch of various size molecules some of which are materially larger than anything that was in the mix before or would be in the mix if the high tem viscosity was a little higher to begin with? If the later, it seems to me that there is little viscosity based rationale for mixing different viscosites of the same brand to get i.e., a slightly higher high temp viscosity....which is all I am really trying to discern. Yes, one could not mix at all and use Mobil 1 0-40w and hope it doesn't shear back and is stable in a 0w-40 spread or use i.e., Amsoil 5w-30 to begin with that has a higher xxw-30 Vis. But, let's say I want to use Mobil 1 xxw-30 SuperSyn. Does the addition of 15w-50 or 0w-40 introduce much larger molecules than would be present if the oil had a slighly higher high temp viscosity (9.7 in this case) to begin with? Sorry to beat a dead horse but I am still not clear on this issue. I am clear, however, on the alternatives of using an alternative brand that has a higher high temp 30 wt viscosity to begin with. Also, the limiting factor or issue is that many OEMs do not recommend anything heavier than xx-30W. Thanks again.

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Originally posted by nick778: So, does this mean that, all things being equal, that if someone mixes baseballs and basketballs, you end up with a melon size molecules or a bunch of various size molecules some of which are materially larger than anything that was in the mix before or would be in the mix if the high tem viscosity was a little higher to begin with?
Mixing two oils with different size base oil molecules is not going to change the molecular structure of the oil. If you mix baseballs and basketballs, what you are going to have in the end is an oil with a mixture of baseball and basketball size molecules. From a viscosity standpoint, this mixture is going to "behave" somewhere in-between where the basketball oil and the baseball oil would have.

Yeah! Whatever ALL of you said. (I think I'm going crosseyed fromt his thread...)

Maybe doing some fuzzy math here, but does this calculation work: Given a 5 qrt sump: Mobil1->4 x 10w30, 1 x 15w50 4 x 9.8 =39.2 39.2 + 17.4 = 56.6 (9.8 and 17.4 are cst@100c #s) 56.6 divided by 5 = 11.3 11.3 would be a medium/high 30 weight. [ February 04, 2003, 07:06 PM: Message edited by: Toy4x4 ]

Nick778, Make things easy on yourself and run some Mobil 1, 0w-40, or perhaps the Redline or Amsoil 10w-40 synthetics. If you mix together a 5w-30 and 15w-50, the more volatile 5w-30 is still going to evaporate at high temps, and the resulting mixture will probably not flow quite a well as a 10wt at low temps. There are differences in the additive chemistry used in these two Mobil 1 formulations and there may well be differences in the particular PAO/Ester basestock blend. Keep in mind what you are creating here is a physical mixture and not a chemical one ....The analogy to baseballs mixed with basketballs is generally the right way to think of this. If I ran into some oil consumption problems with the 5w-30, I might add a quart or two of the 15w-50 in a pinch to slow it down. However for the next oil change, I'd simply use a thicker, fully formulated oil and not try to play organic chemist in my driveway.

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Originally posted by Giles: I thought one of the major benefits of synthetic over conventional is uniformity of molecule size. So by mixing different weight synthetics, one of the major benefits are diminished. For those that are mixing 10w30 and 15w50 would be better off getting a 0w40 weight oil without mixing and be done with it in my opinion.
True, but the problem is that the 0w40 is hard to find in the US, and near impossible to find here in Canada.

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I asked this question to MoleKule in a PM a few weeks back and he mentioned to me that it's not always an exact calculation...
Yep, I remembered MolaKule talking about mixing vis's but the details beyond "all other things being equal" were fuzzy. Didn't want to dig up the old "mixing: good or bad" discussions. I need to check in here more regularly. It's tough to keep up with only a glance now and then.

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