Probably you have a stuck float in the carburetor. This allows gas to fill the bowl and continue into the sump, filling it with gas. Certainly drain the oil of all that junk, but you will need to address the carb problem for a complete fix. There are lots of posts on here about this same condition. Use the SEARCH feature to find the one that fits your problem. This is no big deal, but it sure is an annoying one. A semi solution would be to put an inline shut off valve between the gas tank and the carb. After you install that, close it. Then drain the oil and gas mixture. Refill with good oil. Open the valve. Start the mower and mow the grass. When done with the grass, shut off the valve. No gas can leak into the sump and thin the oil. The carb will need to be addressed at some point. Good luck.