Sounds like you drive your bike on short trips and when this is not the case I guess the cylinder head gasket is broken.

But to be sure, go to a mechanic and get it checked. He can measure the pressure of the cylinder.

If the cylinder head gasket is fine, you only have to change the oil, otherwise the engine can break due to the petrol in the oil.

If it is the cylinder head gasket you have to change it.