Today I try to start my Toro self propel lawn mower and it wouldn’t start. After some troubleshooting I found the gas tank is full of oil. I drain the gas tank, check the oil level and put new gas in. Mower start fine with 1 pull. I mowed for about 15mins and the mower cut off. I begins troubleshooting and found the gas tank is now filled with oil again. I have seen many time where the gas get into the oil tank due to carburetor float issue, etc., but not the opposite. What could cause the oil to get into the gas tank?