I run 10W-40 in the F6B. I understand that multi-weight oils, start out thin...i.e. 10 weight when cold and then as the engine gets to operating temperature the oil thickens to a 40 weight oil...right? So basically, I am running 40 W oil after the first 10 minutes of running the engine.

If that assumption is correct, why do all info. on changing oil says to run the engine for 5 minutes or more BEFORE draining the oil? If the bike has sat for a few days and the oil is at its thinnest plus any heavier particles have settled to the bottom of the pan, isn't that the best condition to drain the oil, not when oil is thick and any anything that has settled at the bottom of the pan is redistributed throughout the motor?

Is heating up the oil before draining an "old school" practice when most oils were single weight dino based?

What am I missing?