You should be treating every tank of gas you run through your motor. Here is the suggested mix from a very trusted Mercury mechanic:
Fuel Additive Recommendations:-Stabilize fuel ANYTIME fuel will not be consumed and replaced within 15 days!!!
-Optis (ALL, including XS & ProXS): Quickleen in EVERY TANK OF FUEL. You CAN combine Mercury QuickCare or QuickStor Fuel System Treatment & Stabilizer (or Sea-Foam if you prefer it) with Quickleen. Use the recommended "dose" of each product per the bottle instructions. For anyone questioning Quickleen recommendation- see Service Bulletin 2001-12 (yes, it's been recommended since 2001!!!).
-Quickleen may be used in ANY Mercury engine- it helps to prevent/remove carbon buildup (primary purpose).
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"Mercury Component Cocktail" (Chemically, this works much the same as the below mentioned cocktail, and requires less components.) This is Don's Preferred "Cocktail":
-1oz Mercury QuickCare per 5 gallons fuel
-1oz Mercury Quickleen per 5 gallons fuel
(If leaving fuel in boat for more than 30 days, be sure to also add Mercury QuickStor).
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"Alternate Cocktail" (IMO, this combination results in clean injectors, direct injectors and spark plugs):
-1oz Marine Stabil or Mercury QuickCare per 6 gallons fuel
-1oz Quickleen per 6 gallons fuel
-1-2oz SeaFoam per 6 gallons fuel