Quote Originally Posted by gurt View Post
One other question : why most of the people change the oil before winterization ? An engine specialist told me that it is much better to not let a brand new oil in the block all the winter long. For me it looks logical to do it like that but most of you do it during winterization. Is there a particular reason ? except the fact that you can run your boat the first day after winter without nothing to do?
There are a number of reasons to change the oil at layup.

Used oil becomes corrosive. This can eat away and pit the soft metals such as bearings and pistons
Used oil collects deposits from combustion (this is why its black when we change it) as well as a little un-burned gas
The crankcase, especially a marine engine, collects condensation through its heat up and cool down cycles. This collect as water in the bottom of the pan.

The best practice is to change the oil and filter and run it one last time for a couple of minutes prior to storage.