If you really wanted to clean it out you can get engine oil flushes. (Although some at least need you to run up the engine!). I gather they thin the old oil to make it come out and have some detergents to help too...
2 stroke is runnier than 4 stroke so I think it will actually run out easier than the 4S, presumably bringing some old 4S residues with it. You might want to measure what comes out and compare to what you put in.?
If in doubt get some cheap 4S oil, replace the 2S with cheap, run it gently on the cheap for a few minutes and then re-replace with your finest brand of 4S. You'd presumably see the blue colour in the oil? Assumign your 2S is blue like mine...?