Tony, this topic has been discussed at length, and I think the general concensus was that it's not necessary.
Run a search using the key word "additive", and you will find more information than you probably wanted to know.
And, as far as how to store the car for the winter, there is a lot of information that you will find if you search that topic, as well.
Just hit the "search" button at the top of the page, and go from there.
Jeff