I had the same problem. I had a guy on RCU recommend Sthil 2 cycle oil and to use the engine recommended oil to gas ratio. I did and was really amazed at how much cleaner it ran. I use fantastic to clean up after wards and it works well. I also heard from several posters that synthetic oil should not be used for engine run-in. Apparently it can cause the rings to not seat properly. After break in it's ok to use. Just what I have read on the subject. Sure there are different opinions. Thought I would pass along.
I am a first time gas engine user so not an expert and learning.