When my oil level is at the recommended level the fuel pump start to pump oil.

Hi,
Generally, that means that the diaphragm in the fuel pump has a hole in it. One side of the pump is exposed to the crankcase, moving the diaphragm inward and outward drawing fuel in and pushing it out through one-way check valves. A hole would expose the fuel system to the oil in the crankcase. Usually it is a B&S #808656 pump.
Peace,
Clarke