I'd use a good quality oil like penrite (because it's readily available and everywhere seems to have it) or they yamaha oil and mix it to what yamaha says. No one knows the engines as well as the people who built them, this is how things normally go wrong some backyarder thinks he knows more than one of the biggest outboard companies and doubles the oil.
Using to much oil can be just as bad as not using enough, clog carbies, create a lot more carbon throughout the motor, foul your plugs and destroy them, make the engine less fuel efficient. Create way to make smoke etc etc.
From my ( a mechanics) point of view is be doing exactly what the manufactures states.