Why heat up multi-weight oil before changing?

[rdbonds]

I found a quick & rough example graph on the interwebs. The lines are not really straight in real life, they're bent a bit, but it is a decent visual.

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[zeus661]

I usually end up changing it after it has sat for a few days. I figure when I park it hot everything eventually settles low. After I pull the plug and drain I sometimes pour half a quart of clean oil to rinse the last few drops laying in the bottom.