And ya thought gas was expensive, welcome to Opti ownership high priced oil and maintenance. Consider shopping around locally at any dealerhsip that sells/services Mercury motors. The price varies quite a bit but if you can find it under $20 a gallon go for it. The average up here is $21+ a gallon.
I would not suggest using anything other then Opti Oil, it's not worth cheaping out on oil for a motor that is worth more then the boat.
Good Luck!
Cass