If you buy a boat motor, it is likely that the company that made the boat had a specific oil in mind when they were testing the motor and getting it to run at its highest level. This is usually an oil that is of their same brand and they will typically recommend it within the manual that comes with the motor when you purchase it. It is best to follow these recommendations so the boat motor works as it should.
With some types of motors, not using this type of oil can make it so that they run less efficiently or don’t run as cleanly. This is definitely true of the Mercury OptiMax motors since they were designed to run more efficiently and cleanly when using OptiMax oil in the motor. Without this type of oil, the motor really cannot run the way that it is intended to.