In the float bowl? If so, it would seem hard for engine oil to end up there on a running engine. Besides, I have seen that kind of varnish/residue before in carbs that have sat with fuel in them. Top end cleaner is about the best stuff I have found to clean up that mess. Sometimes it's called de-carbon, sometimes even tune-up-in-a-can. But it is the only stuff that my outboard service schools would recommend for carb cleaning and it is still the only thing I use on carbs. No carburetor cleaner, it's just too harsh.