Senza has answered your questions. I would only ad that the reason for the castor oil, is to protect the engine in case of a too lean run. As you have noticed, 4 strokes make no sound change when they start to lean out, like a 2 stroke does. The castor oil still keeps lubricating, even when the engine temperatures get out of hand, so caster is good thing to ad for this reason. The reason the muffler is small is because 4 strokes make less sound.