Mercury outboard engines require service every 100 hours or once a year, whichever comes first. This annual service includes changing the engine oil and filter, replacing the gear lubricant, and lubricating all points. More comprehensive 300-hour (or 3-year) service includes checking/replacing spark plugs and changing the water pump impeller and corrosion control anodes.  

Annual (100-hour) service

• Engine oil and filter: Change engine oil and replace the oil filter. Perform more frequently under adverse conditions.
• Gear lubricant: Drain and replace the gear case lubricant.
• Lubrication: Lubricate all lubrication points.
• Spark plugs: Inspect spark plugs and replace as needed.
• Fuel system: Check the engine fuel filter for contaminants.
• Corrosion control: Check the corrosion control anodes more frequently in salt water.
• Power trim: Check the power trim fluid.
• Other checks: Inspect the thermostat, check control cable adjustments, and inspect the battery. 

300-hour / 3-Year service

• This service includes all 100-hour service items, plus:
• Water pump impeller: Replace the water pump impeller.
• Corrosion control anodes: Replace corrosion control anodes.
• Spark plugs: Replace spark plugs. 

Other important points

• First service: 
  A break-in service is recommended at 20 hours or 3 months, whichever comes first.