So...
I did an oil and filter change on Emily ('02 MCS) the other day, and as I was adding the replacement oil, I started wondering; If I mix one part of 5W-30 with one part of 10W-40 (for example), do I obtain an end-result of a viscosity of 7.5W-35?
In other words, is the mixing of different viscosities linear?
I may have to much time on my hands.....

 

It's probably not strictly linear, but I'll bet it's very close.

 

On this thread https://www.northamericanmotoring.co...h-pics-17.html -- (Post 416 by Investigator)

there is a link to a really good article that will tell you more than you want to know about oil.

 

The short answer is No, you will have oil that is 5W-40 (You are adding 5 weight and 40 weight that did not exist in each of the two components, respectively. The 5W-30 indicates the viscosity range from 5 W to 40 W).

BTW, my recollection of thixotropy form Geology class is that it refers to gels/semisolids that change viscosity upon agitation --like clays that become liquid in an earthquake.

 

"The short answer is No, you will have oil that is 5W-40 (You are adding 5 weight and 40 weight that did not exist in each of the two components, respectively. The 5W-30 indicates the viscosity range from 5 W to 40 W)."

Cigarman, that makes perfect sense! I didn't think it through far enough.
And as far as "thixotropy" goes, I was mistooken. I was referring to the study of viscosity and the resistance to shear in a liquid.
My bad.

 

The article Ron-s mini refers to (http://www.focfloridaregion.com/) should be required reading for anyone with an interest in engine oil perfomance. I've read it now a couple of times, and find it very enlightening.