Fellow Boaters, your opinions on this would be greatly appreciated. I agree completely that fully synthetic oil will best protect an engine. I know that all oil (to meet epa specs.) has some synthetic in it. So it seems to me there are three basic classifications of oil "regular" mineral (with some added synthetic), semi synthetic and fully synthetic. I plan to eventually run fully synthetic in my twin 350 Mags. I have just finished putting 20 hours on them - breaking them in strictly by Mercruisers book. In the owners manual Mercruiser recommends a "first" oil and filter change at 20 hours. In the owners manual section on recommended oils Mercury`s number one choice is 20W-40 FULLY synthetic. Soooooo here's my question. I always thought that you should put at least 50 hours on an engine (to allow the oil control rings to seat properly and the bearings to seat properly). I have asked four techs I trust and got different answers some with conditions. I telephoned Mercury HQ and they said I was to follow the manual for warranty reasons. The tech at Merc HQ said the fully synthetic oil was required to avoid phosphorus that will kill the catalytic converters and void the warranty. When I told the Merc HQ tech that I live in Canada and that my new 2013 Mercs were shipped here (the last year we can get non-catalysed) as non-catalysed he said to use fully synthetic anyway. Opinions appreciated.