Most sailboat marine engines need an oil change every hundred miles or so, which for many owners means at the end of the season - since it's better for the engine to have clean oil while laid up for a time. Many owners pay their boatyard for this annual task, which is really quite easy to do with simple equipment during the winterizing process. Here's the best oil extractor to use to remove the old oil.
Another nice improvement you might want to make in the fall to help combat dampness and mildew during the off-season is to install a ventilation buffer beneath mattresses and settee cushions. Read a review here of the the Hypervent system, which is relatively inexpensive and easy to install.
