Not that much. Next time you make a 50 km trip, when you stop check the pipe temperature. Of course the headers will be hot, but I bet you can keep your hand on the end of the pipes. Well, the CB400 have a 4:1, but at least in my CB750 they don't usually get that hot. Add a humid weather and you have a recipe for rust pie!!!
Raul
P.S. I wouldn't care about the oil. Many of the wrenchheads on this forum are very sensitive on the subject, and will put fresh oil before the winter. Of course it will make no harm, but it doesn't mean used oil will do harm either. Unless your bike is almost due for oil change, I would keep it in. Contrary to many people, I don't enjoy maintenance tasks like oil changes so I do the strictly necessary changes.