# Viscosity of oil mixture

If I mix motor oils of different viscosities, is the effective viscosity of the mixture a weighted average of the viscosities of the oils in the mixture?

For example, consider a mixture of 1 part SAE 10W30 to 3 parts SAE 20W50.

25% SAE 10W30 + 75% SAE 20W50

= (0.2510 + 0.7520) W (0.2530 + 0.7550)

= 17.5W45

So this mixture of 1 part SAE 10W30 to 3 parts SAE 20W50 has effective viscosity of 17.5W45.

Is that how viscosities of mixtures are calculated?

No, that’s NOT how they are determined.

If you, for some stupid reason, wish to mix multi-viscosity motor oils, it is IMPOSSIBLE to CALCULATE what the resulting mixture might be. You will have to set up a test stand and MEASURE the resulting viscosity…There are simply too many variables at work to predict the characteristics of the mixture.

It doesn’t mix to create a different compound. Instead you’ll have a mixture of 10w-30 and 20w-50 weight oil mixed together. Not combining to make a different compound.

www.carbibles.com has a great section on oils. I recommend a visit.

Mixing 10W oil with 20W oil will provide a viscosity somewhere between the two, however it may not be able to be calculated in the manner you seek.

Be aware that adding oils with different amounts of viscosity modifiers (“30” vs. “50”) complicates the problem.

www.carbibles.com has a great section on oils. I recommend a visit.

I took a look, but did not see anything covering the mixture of various weights of oil.

Be aware that adding oils with different amounts of viscosity modifiers (“30” vs. “50”) complicates the problem.

“Complicates the problem”, as in making it difficult to determine what the viscosity will be, or as in bad for the engine?

As in difficult to calculate.

Viscosity modifiers are actually microscopic coiled polymers that expand when subjected to heat and help the oil retain its resistance to flow (viscosity). Different numbers (“30 vs 50”) indicate different amounts of modifier added, but the modifiers in both are the same stuff and mixing the two presents no problems. It becomes a homogenious mix, consistant throughout.

A word about modifiers: they don’t lubricate as well as the oil, so as the modifier number rises, the lubricating properties of the oil suffer. 10W40 does not lubricate quite as well as 10W30, but it does maintain its resistance to flow better at higher temps. This is important because (for example) the crank surfaces are kept away from the crank bearings by a barrier of pressurized oil being forced through the space. If the oil flows through too easily, the pressure is more dificult to maintain (or becomes lower) and the protection suffers. Oil formulations are a delicate balance.

It isn’t simple. But, in summary, the oils and their viscosity modifiers all mix just fine and will be perfectly fine for the engine (as long as both are within the recommended ranges), it’s just that determining the resistance to flow of the resultant mix may not be as direct as adding and dividing…mostly because of the modifiers.