Hi KB. I suggest that the easiest way to remove the oil residue is to clean the original carburetor properly, with a spray can of carburetor cleaner. They are supplied with a long, small-diameter tube so you can blast through each drilling and recess in the carburetor. Do not use it on the elastomer parts - it will age them somewhat - and blow the residue of the cleaner from each passage with compressed air.
Many mower carburetors are exposed to 2 stroke mix at some time, but it does them no lasting harm and can usually be sorted out just by switching to clean fuel and putting up with some mildly erratic behaviour for a short time. If you are having issues that last beyond using, say, one tank of clean fuel, I do not think the problem is due to 2 stroke mix. More likely there is water or gum in the carburetor, and it needs to be cleaned out.
