Hello,
My name is Dane and I am going to help you solve your problem.
Your problem stems from a float valve needle that either has gotten a piece of debris in between the needle and the seat, or has gummed up due to bad fuel. This allows the Carburetor bowl to overflow and actually can allow fuel to bypass the rings and get into the crankcase, mixing with the oil. To verify this, remove your dipstick and check the oil level. If it's high then you have this problem. Also you can check and see if the oil is extremely thin or may have a gas odor to it. If any of these problems are found, do not run the engine. After this problem is corrected you will definitely have to change the engine oil.
Now, to your main problem. You are going to have to remove the Carburetor from the engine. Pay close attention to how it comes off and maybe draw a picture if you need to. Now you have to carefully remove the float bowl. There will be a pin that holds the float onto the Carburetor and it needs to be removed. Be careful not to bend the float. Pay close attention to how the float needle connects to the float. Next spray Carburetor cleaner into the hole where the float needle goes into and any other dirty areas. You should not spray Carburetor cleaner on the rubber o-rings or gaskets.
Once everything looks nice and clean, place the float needle back into the float and set the complete assembly (needle first) back down onto the Carburetor and insert the pin.
If any gaskets are torn or worn, replace those also as you put the Carburetor back onto the lawn mower. Check your fuel hose for cracks or tears also. When you connect the fuel line, check to see if fuel still flows as it did before. If not, change the oil, and check your spark plug for fouling, you're done! It could take about 3-4 minutes for the Carburetor Bowl to fill with gas before you see it running out. Well, I hope I