i got a 01 toyota corolla and i got problems with it losing oil. i checked every seal and gasket and nothing not a drop. i changed out the valve cover gasket because there was some oil in the back of the cover. with the cover changed no oil leaking from there and looked under the car nothing there. i checked the oil when i went to go put gas in and it was very low as in barely any. last time i got the oil changed was about a month ago and it was good up until a few days ago i guess. so what i am asking is any ideas on where and how to fix? i also need an O2 sensor fixed on it so idk at this point.