Morgans does a lot of engine testing, not just on model airplanes but also go kart engines an moter cycles, I trust their recomendation over engine manufactures. Adding petroleum after run oil does not get rid of the remaining model fuel, in fact if you have a high nitro fuel the after run oil will float on top of the fuel. It is also lighter than water. I have always thought that petroleum after run oil is sometimes a cause not a cure for corrosion. The best after run oil is plain old bean oil.