It is advised to warm up the engine before oil change to ease up the oil drain. For a multi grade oil say 10W-30 at cold temperature the viscosity is 10(thinner) and if the engine warms up the viscosity would be 30(thicker) so it would flow more easily at colder temperature when the engine is cold isnt't it? Is this the correct understanding?
No, the understanding is incorrect. All oils reduce in viscosity as the temperature goes up. The 10W-30 just means you have grade 10 viscosity when cold, and grade 30 viscosity when hot. Grade 10 oil when cold is still thicker than grade 30 oil when hot. According to Wikipedia:
The viscosity of a multi-grade oil still varies logarithmically with temperature, but the slope representing the change is lessened.