# Viscosity calculator not linear?

#### Klutch9

Was playing around with the Widman viscosity calculator, where you enter the cSt for two different oils, and input the percentage of each oil. I found that even when you have half of each oil, the resulting viscosity is not half of the two added together and divided by two. This is true of the 40c and 100c calculation. Why is this? I can only assume that something there isn’t linear but it doesn’t describe what/how.

When mixing two different oils nothing is linear and it's anyone's guess what the outcome is.

Too bad everything in life wasn't linear ...

Too bad everything in life wasn't linear ...

Nothing is linear.
Nothing is absolute, we call it tolerances or ranges.
All those number in oil spec are within tolerances to a standard.
They are not exactly set to ONE number.
Then, these calculators are working with formula based on a model (again not exact).
In reality, the result of mixing oil will be dependent on the exact number within that tolerances.

Confuse yet????

Viscosity blending is plotted on a log/linear scale.

Tom is correct. Viscosity vs temperature is non-linear. Mixing two different viscosity oils at a given temperature is highly linear. This is consensus in science.

### Some Models for kinematic viscosity​

The effect of temperature on the kinematic viscosity is described by a number of empirical equations.

The Walther formula is typically written in the form

In lubricant specifications, normally only two temperatures are specified, in which case a standard value of
= 0.7 is normally assumed, and
and
are empirical parameters specific to each liquid.

The Wright model has the form

is often a polynomial fit to experimental data, has been added to the Walther formula, and
and
are empirical parameters specific to each liquid.

The Seeton model is based on curve fitting the viscosity dependence of many liquids (refrigerants and lubricants) versus temperature and applies over a large temperature and viscosity range:

is the zero order modified Bessel function of the second kind, and
and
are empirical parameters specific to each liquid. [Seeton, Christopher J. (2006), "Viscosity-temperature correlation for liquids", Tribology Letters, 22: 67–78, doi:10.1007/s11249-006-9071-2]

"Handbook of Physics and Chemistry"

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Are we still talking about the same?

### Some Models for kinematic viscosity​

The effect of temperature on the kinematic viscosity is described by a number of empirical equations.

The Walther formula is typically written in the form

In lubricant specifications, normally only two temperatures are specified, in which case a standard value of
= 0.7 is normally assumed, and
and
are empirical parameters specific to each liquid.

The Wright model has the form

is often a polynomial fit to experimental data, has been added to the Walther formula, and
and
are empirical parameters specific to each liquid.

The Seeton model is based on curve fitting the viscosity dependence of many liquids (refrigerants and lubricants) versus temperature and applies over a large temperature and viscosity range:

is the zero order modified Bessel function of the second kind, and
and
are empirical parameters specific to each liquid. [Seeton, Christopher J. (2006), "Viscosity-temperature correlation for liquids", Tribology Letters, 22: 67–78, doi:10.1007/s11249-006-9071-2]

"Handbook of Physics and Chemistry"
I think I get this. My main question was regarding mixing two oils at a specific temp. It would stand to reason that if I mixed two oils at a 50/50 ratio, one having a 14cSt viscosity at 100C and the other having a 10cSt viscosity at 100C, I should end up with 12cSt. But I don’t, it’s always a little different than you’d think.

I think I get this. My main question was regarding mixing two oils at a specific temp. It would stand to reason that if I mixed two oils at a 50/50 ratio, one having a 14cSt viscosity at 100C and the other having a 10cSt viscosity at 100C, I should end up with 12cSt. But I don’t, it’s always a little different than you’d think.
You can mix them but have them tested to determine the final viscosity.

Some extra variables that are thrown into the discussion when mixing are:

1) Oil A may use a different type of VII than oil B, altering the final expected viscosity,

2) Oil A may have a different mix of base oil viscosities than Oil B, altering the final expected viscosity,

3) Oil A may have a different mix of base oil types than Oil B, altering the final expected viscosity.

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It's misleading. Some might conclude that mixing two viscosity oils would result in something highly unexpected.
That isn't the case. When mixing e.g. KV100 12 cSt and 14 cSt you will get something near 13 cSt. It doesn't really
matter for personal use if you'll get 12.9 cSt or 13.1 cSt. That said, I wouldn't mix two very different oils anyway.
If I'd mix oils I'd only use oils w/ very similar base oils, same add pack and adjacent viscosities (5W-30 + 5W-40).
.

That said, I wouldn't mix two very different oils anyway.
If I'd mix oils I'd only use oils w/ very similar base oils, same add pack and adjacent viscosities (5W-30 + 5W-40).
.
why? would it really cause any real-world harm to the engine?

the reason I'm asking is, I might be getting some oil on clearance.

I use 0w20 for both of my 2GR-FEs, my final viscosity goal is to be within 0w20 spec.

the oil used would be Castrol edge 0w16 (5 quarts) and 10w30 (1.5 quarts) per oil change, the only reason I'm mixing is to keep the cost down for the oil change, yes I know I could just buy a 0w20 and do that but that viscosity is never on clearance.

why? would it really cause any real-world harm to the engine?
If the oils you are mixing are API licensed or have otherwise passed ASTM D6922 then no harm will result from mixing as they must be miscible. However, there is no guarantee the resulting mixture will meet any license, specification or approval any of the oils originally held. The winter rating is not guaranteed either if that is important to you.

If the oils you are mixing are API licensed or have otherwise passed ASTM D6922 then no harm will result from mixing as they must be miscible. However, there is no guarantee the resulting mixture will meet any license, specification or approval any of the oils originally held. The winter rating is not guaranteed either if that is important to you.
Exactly. As @Shannow would frequently say, the miscibility standard means that the oils won't split like mayonnaise. It does not however mean that the resulting product will retain the performance of any of its constituents nor does it guarantee that the Winter rating won't be negatively impacted.

The miscibility standard is not a license to mix, though some seem to take it that way, extending it to doing so with impunity, with the potential for the resulting product somehow being an improvement. Its existence is to ensure that you can add another oil in a pinch, if in a situation where that's necessary, and nothing is going to catastrophically fail.

If the oils you are mixing are API licensed or have otherwise passed ASTM D6922 then no harm will result from mixing as they must be miscible. However, there is no guarantee the resulting mixture will meet any license, specification or approval any of the oils originally held. The winter rating is not guaranteed either if that is important to you.
winter rating isn't important as the vehicle is parked in a heated garage, only the final operating viscosity must be of a 20 weight.
Exactly. As @Shannow would frequently say, the miscibility standard means that the oils won't split like mayonnaise. It does not however mean that the resulting product will retain the performance of any of its constituents nor does it guarantee that the Winter rating won't be negatively impacted.

The miscibility standard is not a license to mix, though some seem to take it that way, extending it to doing so with impunity, with the potential for the resulting product somehow being an improvement. Its existence is to ensure that you can add another oil in a pinch, if in a situation where that's necessary, and nothing is going to catastrophically fail.
just trying to make sure the bearing will have adequate lubrication at operating temperature, that's all.

winter rating isn't important as the vehicle is parked in a heated garage, only the final operating viscosity must be of a 20 weight.

just trying to make sure the bearing will have adequate lubrication at operating temperature, that's all.

Well, it's quite possible you won't have a 0w-20 with your concoction, most likely it would be a 5w-20, worst case it may be a 10w-20. With a 100C visc of ~8cSt. You also don't know what the HTHS is going to be, but it may not be the 2.6 of a formal 0w-20/5w-20, it could be lower, like around 2.4-2.5, which is what matters to your bearings. Energy Conserving xW-30 oils tend to be on the low end of the HTHS scale, around the 3.0 mark, so if the 0w-16 is 2.3 and the 10w-30 is 3.0, you'll be around 2.45, which is below the 2.6 spec'd for the engine if it calls for a 0w-20/5w-20.

Well, it's quite possible you won't have a 0w-20 with your concoction, most likely it would be a 5w-20, worst case it may be a 10w-20. With a 100C visc of ~8cSt. You also don't know what the HTHS is going to be, but it may not be the 2.6 of a formal 0w-20/5w-20, it could be lower, like around 2.4-2.5, which is what matters to your bearings. Energy Conserving xW-30 oils tend to be on the low end of the HTHS scale, around the 3.0 mark, so if the 0w-16 is 2.3 and the 10w-30 is 3.0, you'll be around 2.45, which is below the 2.6 spec'd for the engine if it calls for a 0w-20/5w-20.
thank you so much for that, then maybe Castrol edge 0w16 (4 quarts) and 10w30 (2.5 quarts) per oil change, is it possible to get a "ballpark" HTHS figure using a worst-case scenario for these oils?

I know without testing there isn't a way to know, but is there a way to at least estimate?

thank you so much, I truly appreciate this.

thank you so much for that, then maybe Castrol edge 0w16 (4 quarts) and 10w30 (2.5 quarts) per oil change, is it possible to get a "ballpark" HTHS figure using a worst-case scenario for these oils?

I know without testing there isn't a way to know, but is there a way to at least estimate?

thank you so much, I truly appreciate this.
I expect you'd get close plugging the HTHS numbers into the visc calc instead of the KV100 ones.

I expect you'd get close plugging the HTHS numbers into the visc calc instead of the KV100 ones.
hmm, that means I would need at least a 50/50 mixture to get close to the 2.6 HTHS value for a 20 weight for my concoction.

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