ORIGINAL: FLYBOY
Never used it, never will. I use it if I am going to store an engine, but burning out the extra fuel and then adding oil makes no sense. The nitro evaporates from the fuel before you get it put in the car and all that is left is oil. That does the job just fine. My friends that burn out the fuel end up changing bearings, I don't. I figure, if it works, don't change it.
Actually, when the fuel evaporates, it causes the metal of the engine to cool, which will then draw moisture out of the air like "sweat" to a beer bottle. The after run oil is supposed to coat and protect the engine components, bearings in particular, from the moisture collected from fuel evaporation.
Fuel with castor oil tends to coat and protect engine components better than fuel with only synthetic lubricants. Your friends who burn off all of their fuel but later end up changing bearings are probably running 100% synthetic lubrication fuel and then failing to use after run oil.
If you run fuel without castor oil, you will want to use a fuel blend that contains after run oil components or to use after run oil periodically to protect your engine bearings. If you run fuel with castor oil, you don't need to use after run oil between flights all that frequently, but the after run oil should be used when storing your engines to help prevent the castor oil from drying out in storage and "gumming up" your engines.
There is no harm that can come from using after run oil. Not using it, however, can lead to rusty bearings or engines gummed up from being stored too long.