To avoid possible confusion before I comment fully, what specific "it" are you referring to?
The "It" I'm talking about refers to the subject of "thinner oil cool better". Specifically in the application of heat exhangers (oil coolers) since Post #9 specifically brought that up, and that's the main focus I was discussing.
But there's also the heat generation aspect of oil due to viscosity (ie, thinner oils don't heat up as much from shearing), which is a different factor than the basic heat transfer properties based on viscosity alone involved with just an oil cooler (fluid thermal properties dependent). As a couple of the links I posted shows, the formulation of the motor oil with the same viscosity rating can also change the Cp (maybe more so than just viscosity alone), so when you really start looking at it closely it's a bit more complicated than just saying "thinner oil cool better" because there are multiple factors going on when you consider all the thermal dynamics of an ICE oiling system going on.
I think people who see that the oil temperature might run slightly cooler in an engine that's pushed really hard are seeing more of the difference from the effect of the heat generated from shearing than from the difference in the oil's cooling performance. Even though a thicker oil might cool slightly better due to higher Cp (release more heat) , it is also heating up more due to shearing (compared to a thinner oil), which could still result in a net increase in oil temperature than a thinner oil. That would give the illusion that "thinner oil cool better".
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