Something in my gut tells me this is a stupid question and the answer is obvious. But, my gut didn't give me the obvious answer yet.

In this day of multi-weight oils, (20w50), why do we drain the oil when it's hot and therefore more viscous (50w)? Why not drain it when it's ambient temperature and less viscous to drain easier (20w)?

 

Try a test. Take a quart of oil and pour half into a second container. Now put one container in the refrigerator. Wait an hour or so and take it out. Now take both containers and simultaneously pour them into a third container. Which one pours faster?

 

Hot oil is much less viscous then cold oil. Duecedog has it right.

 

The test makes sense but the process is sort of lame. No one changes their oil in a refregerator

 

Things move more fluidly when they're hot. Observe:

 

In addition the contaminates are better suspended in the oil.

 

All the junk is suspended in the oil.
As it warms up, the contaminates are taken off the internal parts and flow with the oil. If you just drain the oil when cold, it just will sit in the bottom of the engine and tank.

And the oil does flow faster taking the junk with it.