you should clean it up as good as you can then get a good rust converting primer and cover it up. supposedly the primer will stop all rust and keep it from rotting away the underbody. sure it is a lot more work then just spraying they underneath with oil or chain lube but it wont attract as much dirt either. the oil is gonna turn to grime and make it ral hard to clean and it will be messy to work on if the need arrises. every summer i go under my truck and clean it up, the paint it with some flat black to keep it looking nice. i dont have any real issues with rust because of this.