The after-run oil is purely optional, but the Marvel Mystery Oil won't cause you any problems at all, and will coat the internal parts a bit to keep humidity from attacking the parts. The aluminum parts of the engine are pretty well invulnerable, but the bearings themselves, the little balls, are prone to get small spots of rust and corrosion that will cause them to fail at high speeds. I just don't like to leave an engine for any length of time with no protection on the parts.

David