Fuel in oil

You have Leaking Carburettor float valves.

With the engine stopped, the carb floats overfow out the Jet ports into the throat of the carburettor.

This fuel finds its way into a cylinder that just happens to stop with an open intake valve.

As the cylinder fills with fuel, it also slowly leaks past the gaps in the piston rings into the Crankcase sump where the Oil accumulates.

This is the same leak process used to purge the Oil back to the Oil tank.

Then you turned the prop you forced the Fuel into the Oil and the excess Fuel/Oil Mixture overflowed the Oil Tank.

You will need to check the weight of the floats and also perform a complete Oil change.

In Hindsight, it is always a good idea to close the Fuel valve between flights to prevent this issue.