Question for a petroleum chemist/engineer.....

[pippo]

I heard a while ago that if you want to get say, 15w-40 oil, you can simply blend/mix some 10w-40 with 20w-40 (such that the first #'s 10 + 20 divided by 2 equals the 15). Likewise, if you wanted say, 15w-40 you can achieve by blending 10w-30 with equal amount of 20w-50.

 

In other words, they are miscible, and the equal amounts behave linearly when mixed. Is this true?

 

Thanks

[Essay]

I don't know specifically, but mixing different weights of oil (being okay for "single weight" oils) was the common folk wisdom from back in the day, except that you shouldn't mix oils from different brands/formulations. But I don't recall what they said about the mixing of variable viscosity oils, so sorry.

 

Though all that was back before high-tech sensors were added to engines, or oil companies became more innovative with their different formulations; so I'd be more worried these days about trying to get creative with my resources. I'd guess that mixing the wrong two additives could cause problems.

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[John Cuthber]

It was probably true when engine oils were just petroleum fractions.

But now, with viscosity control additives etc. I wouldn't expect it to work properly. It would not surprise me to vind that the viscosity of the mixture was not even somewhere between the viscosities of the two starting materials.

I'd do it in an emergency, but I'd flush the mixed oil and replace it as soon as I could.

[Enthalpy]

Even with simple and similar compounds, viscosity wouldn't combine as a mixing proportion. Some formulae exist, like combining the logarithms as the mixing proportion. With very different viscosities, the blend is thinner than combining linearly. But with 10w and 20w, this difference is negligible.

 

-40 implies additives to control this viscosity index, so the viscosity drops less at heat. Things like polyglycols are used. If the two oils use very different additives, there might be surprises. And of course, you have incompatible lubricants or hydraulic fluids, which may even not mix at all, like oil distillates and water-based hydraulic fluids. There are phosphates, polyglycols and several more families, very different from petroleum.

[pippo]

Thanks, people. True, today oil companies are using different recipes/additives/etc, although, I understand it is not that high tech. Only thing is, as been mentioned above , blending different brands could create a "bad"/unintended reaction and get one in even more trouble than it's worth.

 

Also, while I was anxiously waiting for some advice here, I tried calling around- contacted 2 places- one was a blender/supplier (a tech guy though, not just a sales rep), other was Mobil tech service. Blender said blend is OK/no big deal, and even can average the numbers on the weight (not viscosity). But the mobil tech guy- sheesh, talk about protecting company secrets- said "no you cant do that, otherwise we can be liable.........." and more protective/defensive mumbo jumbo which I am used to, and expected 100%. Why would he say otherwise.

 

Thanks again.