I think doubling the amount of oil is a mistake. The plugs will foul quicker, there will be more carbon build up which will actually reduce the life of the motor and there will be less complete combustion which means you'll end up dumping more unburnt oil & gas into the water. Yamaha would not spec 100-1 if it wasn't what was best for the motor. I have a lot of hours on my '06 motor at 100-1 & I've had no problems.
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