Added fuel additive to the engine oil

If you mean the stuff like in the image? If you haven't run the engine, the easiest thing would be to change the oil using the correct oil. If you have run the engine do the same thing, except I would probably change it and the filter twice. It's very similar to the chemicals used to flush oil systems, but I wouldn't leave it in the engine. I would probably buy a cheap oil that meets the specs of the engine, then change the oil and filter. Start the engine and let it run a minute or so, then change the oil and filter again using good oil. That should remove most of it from the oil system.