[Alan Wheeler]

The main reason I like synthetic oil in a Marine engine is the ability to protect from corrosion, even if the engine has been left idle over winter. If salt water does get into the oil, the parts are protected. Very highly polished surfaces lift cams etc, are very suseptible to rust pitting. A film of synthetics can protect those surfaces. Also synthetics tend to be a better hydrophobic fluid than mineral oil. It is much harder and in some cases just impossible for the synthetic to mix with water at all. So you don't get that sludge build up around criticle areas of the engine. Water simply "boils" off and tends to go to the top of the engine.
The next that the synthetics tend to be far more stable. They do not contribute to sludge and varnish build up in the engine. They also have excellent detergent agencies to keep the engine clean.
There are many very good Synthetics out there. The reason I like Amsoil is not so much for the oil, although I have so far had fantastic results, it is that they have a very very high quality filter available. If you don't use the oil, at least get your hands ont he filters. You could use the best oil created by the hands of God, but without a good filter, it still gets dirty and contaminated by metal which causes wear. A good filter that can minimise those particles is essential.