When switching from conventional to synthetic oil in your outboard motor, there are a few potential issues of which you should be aware. First, recognize that once you start to use a synthetic oil, it’s near impossible to go back to a conventional oil. This is because synthetic oils swap themselves with the plasticizers in the seals; should the synthetic oil come out, the seals will shrink and crack because the plasticizers are gone. Depending on the type of outboard motor you own, using a full synthetic, like evinrude XD-100, can be better than a partial synthetic like evinrude XD-50. You should always be mindful to check with the manufacturer or consult an expert before switching.
Amongst synthetic oils, evinrude oil is one of the most renowned brands, with a proven track record of successes, and much preferred by outboard motor owners.