For a long time I swore by using mineral oil in my outboard motor. Year after year I experienced zero issues with the particular brand of motor oil I used. I’m not sure if the brand changed their formula or if my engine simply succumbed to its age, but the outboard started smoking far more than usual.
To reduce the outboard smoke, a mechanic friend of mine suggested I use synthetic oil outboard motor instead. I was reluctant but eventually did make the switch. Much to my surprise, the synthetic oil did the trick! From that point on I didn’t have any further issues with a smoking outboard.