From what Mercury says, you should add fuel stabilizer to the fuel along with a half liter of your premix oil. Change the fuel filter and fuel/water separator if equipped. ( not sure on the carbed 210, I have the DFI 200 ) Run the engine until you feel the stabilizer and oil have reached the carbs, then use fogging oil down the intake. The last thing to do is change the jet pump oil.
In the spring top the fuel tank up with fresh fuel, and once you burn off the tank containing the stabilizer and oil change the spark plugs.
I also remove the battery and keep it in the heated shop on a battery tender.
Hope this helps.
Aaron