If you find gas in the oil, the first thing you need to do is change the oil. To fix the problem you'll have to pull the carburetor. Go to your local authorized small engine shop and get a Needle Valve and Seat kit. (Kohlers, Hondas, and Kawasakis will have the Needle Valve and Seat come in an entire Carburetor Kit (Includes all gaskets, o-rings, washers, etc). Unfortunately, with those brands, that is the only way you can get a Needle Valve and Seat.)
The following instructions will help you fix the problem:
1. Stop the fuel flow to the carburetor by removing the fuel line.
NOTE: MAKE A NOTE OR DIAGRAM OF HOW THE GOVERNOR LINKAGE IS CONNECTED TO THE THROTTLE ON THE CARBURETOR.
2. Remove the carburetor.
3. Over a pan suitable for containing gasoline, remove the fuel bowl nut and the fuel bowl.
4. Turn the carburetor upside down and remove the float and needle valve by pulling out the float pin.
5. Remove the needle valve from the float.
6. Remove the needle seat by pulling it out with a piece of coat hanger wire or blow the seat out with compressed air from an air compressor.
Warning: IF YOU CHOOSE TO BLOW THE SEAT OUT WITH AN AIR HOSE, POINT THE BOTTOM OF THE CARBURETOR AWAY FROM YOURSELF, OTHER PEOPLE, PETS AND THE WALL AND WEAR SAFETY GLASSES.
Blow the seat out by blowing air through the fuel fitting. Keep blowing until you hear a popping sound.
7. Get the new seat. There will be grooves on one side of the seat. When installing the new seat, spray some WD-40 on it and install the seat so the grooves face down in the carburetor, (The smooth part of the seat will be facing you). Press in the seat with a flat punch until the seat bottoms out.
8. Replace the needle valve with the new one in the kit. (Assemble the wire clip to the needle valve) On Tecumseh engines, the long end of the clip must face the Choke or air cleaner side of the carburetor. If you fail to do this, the needle valve will not seat. With Briggs and Stratton, Kohler, Kawasaki and Honda, it does not matter which way the end of the clip goes.
Note: On Briggs & Stratton 8-22 HP engines, the seat is brass and pressed into the carburetor. It can only be removed with a Tap and a bolt. It's highly recommended that you take the carburetor in to a local shop and have them pull the seat out for you and reinstall the new needle valve and seat kit. Brass is a soft metal and very easily damaged. They know what they're doing.
9. Assemble the float and needle valve to the carburetor and check the float level. Make sure the float is parallel to the carburetor body. If you have a pressure tester, connected it to the fuel fitting. This will tell you if the seat will hold gas or not. If the seat can hold 5 P.S.I., then it is ok. Install the fuel bowl and nut.
You should also check for gas in the float. While the float is removed shake it & listen for gas sloshing around inside it. It you hear gas sloshing around inside the float you should replace it too.