Wait... what? Air viscosity increases with increasing temperature?
 

How do we reconcile the following two viewpoints?

Air viscosity increases with increasing temperature:
https://en.wikipedia.org/wiki/Viscos...osity_of_a_gas

Drag is linearly dependent on density so if you heat the air by 1%, density decreases by 1% and drag also drops by 1%.
https://www.physicsforums.com/thread...e-drag.804563/

So which is it? Does hotter air increase air viscosity (and thus drag), or does hotter air decrease air density (and thus drag)?

You should have read further in that topic on physicsforums.
Both statements are true, it's just that the density effect is much larger than the viscosity effect.

Yup, I see that now. Is there a temperature above which this happens, or does it happen all the time? I saw an equation on the Wikipedia web page, but it wasn't clear to me.

wait
 

You might want to avoid wikipedia and search out a density altitude calculator online to put you in touch with reality.
We're not interested in 'viscosity',although 'density' is an issue if you want to drive yourself mad.