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.
When i had the power head on my 225 opti replaced, the mechanic told me to just use Premium Plus in it. He said it was just as good as the synthetic. In fact that dealership doesn't even carry the opit oil anymore.