outboard maintenance schedule

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i have an 05 optimax that requires maintenance every 100 hours or yearly whatever comes first.

i put about 50 hours a year on mine.  as long as my motor doesnt sit for extended periods of time,

why cant i wait another year until i hit the 100 hour mark?  

i dont recall a maintenace schedule for cars telling you to perform maintenance other than when you hit a certain milege.  so as long as the equipment is used regularly and not sitting why is an outboard schedule any different.

i am looking for something other than it might void the warranty,  thanks  

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Oil canbreakdown over time.  Auto manufacturers typically recommend changing oil every x miles or x months.  I assume this logic could apply to your lower unit oil.  What maintenance is recommended very year?