What causes the gas to mix with the oil on a lawn tractor even with a new carburetor?

Check to see if you have sand or dirt in the fuel tank. Might be a new carburetor on it but if the sand gets into it from the old gas, it will prevent the inlet needle to close and fuel will continue to flow. I would pull the fuel supply hose off and drain the fuel tank, wipe it out with a clean rag and a long screwdriver, replace the fuel filter, and pull the carb bowl and clean.