my dad have always told me to change my oil when the engine have reached operating temperature. his reason is that oil flow better when its hot, therefore, more oil will drain out when changing the oil, so i usually change my oil right when i just got back from somewhere, and when the engine is hot. however, i'm thinking that since i'm using multi-grade oil(5w-30). oil is thinner when it's cold and thicker when it's hot, so changing it when the car been sitting for a few hours or over night will get more oil out. plus, the oil is already seattle down into the oil pans. what do you guys think? do you change your oil when the car is hot or when the car has been sitting for awhile and completely cooled down?