With many outboard motors, the operator actually has a bit of choice when it comes to buying oil for the motor. It isn’t the end of the world if you have to use the cheaper brand of oil a few times because your local marina has run out of your favorite brand. Some people even manage to run these outboards only on cheap oil for the majority of their lives.
For some more special outboards, like Optimax outboards, it is important to buy oil that is made for this engine. Since the engine has special requirements that other oils might not be able to provide, you should always stick to using Optimax oil when you are operating this outboard. Using another type of oil could cause damage to the outboard in the long run.