check ur engine for leakage, park it somewhere clean over night and check the floor for oil spots in the morning. or maybe the engine is burning very little amount of oil you might not be able to tell if it the engine is burning oil just by looking at the exhaust,Check the exhaust for an unusual smell of burning when the engine is running Other than that there is no explanation about the engine losing oil because it is a sealed system.and synthetic oil is nothing special.all the automotive oils you buy are synthetic oil(man made oils)and mineral oil is the one that is natural.like coconut oil and olive oil.hope it helps