At what average temperature do you change from 20W50 to 10W40? Or do you?

 

Well synthetics have a significantly lower pour point than a conventional so it really depends on the oil.
Out of habit I go thinner when temps are 10c or less when I start my bike.
I change the oil in my vehicles around the same time to a 0w-30.
In the bike I go to a 5w-40 euro spec oil mixed with rotella t conventional.
It's thin enough at start up to flow easily and thick enough at temp to keep my valvetrain quiet.
If you want a quiet valvetrain don't use mobil 1 in any flavour. Some members don't notice the clatter or marbles bouncing around but you will find most guys who have used it and switched,it was for that reason.
And I'm confident that there are more people who notice
the noise the marbles then people who don't.
This is just my experience. Yours,and others may vary

 

When an oil's weight changes it's more from shearing then temp. Oil's tend to thicken up when exposed to high temps. Oil used in "shared sump" motor setup's tend to shear from being run through the motor and the trans gears. While most Harley's have separate oil between the trans and motor, alot of people run gear oil in their trans, which is more resistent to shearing. Some motor oils will also shear a grade or two based on a combination of heat and how many gears in there is in the motor. Chains , cam gears, oil pump gears are what helps chew up the oil.,,

 

Being that your from Illinois, you could have two riding seasons. According to my manual Average temps above 40 deg use 20w-50, below 40 use 10w-40. I have ridden mornings below 40 with 20w-50 just let the bike warm up a bit first.

 

35-40f