After run oil is a good idea if properly done. It takes more than a few drops. If you are using any nitro in your fuel, there will be some residue in the crankcase (even if you run the engine dry) that can and will combine with water in the air to for an acid that will attack the bearings. You need to put enough oil in the engine to displace any gunk or residue left in the engine (at least a teaspoon full) then turn the prop over a few times with the nose low to distribute that oil over the surface of the bearings. A good after run oil is Rislone (the one in the yellow container) which is available at most auto parts stores, Walmart, etc. and it is much less expensive than after run oil from the hobby shop. Any air tool oil will also work well. Marvel mystery oil is not the best choice as it has little or no protection from moisture. Marvel mystery AIR TOOL oil is okay. After oil will definitely not hurt your engine but not using it may.