Gear Oil for Yamaho Outboard

[Gumbo]

You shouldn't use 100% synthetic oil in a new engine because it lubricates too well to prevent a proper break-in of the motor. But the seal rumor is a myth. What can happen is if you add 100% synthetic to an older motor, the synthetic oil will find any weaknesses since it is less viscous and it can leak. But if you add it to a newer motor (after break-in), then there's not a problem.