To use or not to use afterrun oil depends a lot on your location, in damp climates it will almost certainly help and certainly will not hurt anything. WD40 is a NO NO in engines, especially older ones that may have accumulated some rust anywhere in the interior cavities. It is not a lubricant but is a solvent. Another good substitute is any air tool oil as stated above or Rislone or ATF, all much less expensive than the after run oil sold as such and performs just as well. As to how much to use, more than a few drops. You need enought to coat the innards of the engine, especially the crankshaft and conrod bearings, usually about a teaspoon full put in thru the carb on 2 cycle engines or into the crankcase vent on 4 strokes. Putting it down the carb on a 4 stroke does little or no good.