The best thing to do is start fresh and clean the unit up, so you can get an idea where the fresh oil is coming from. Here's a few questions you need to ask yourself to help determine the problem. I probably missed some questions, but these will give you a good start. So lets review them. The latter would normally have showed up when the unit was new though.
All modern engines run with a slight vacuum in the crankcase.
This is maintained by the crankcase breather valve. If this valve is not functioning or is installed wrong, you can be building a pressure in the crankcase. Many engine gaskets, seals and governor shafts are not designed to handle a pressure, so out comes the oil. Oil that is to thin because of gasoline dilution can cause all kinds of leaking problems and smoking conditions as can water in the crankcase, which causes foaming. In the Summer heat many engine manufacturers recommend 30W oil.