Changing Engine Oil of 15 or 20 HP 4 Stroke Mercury Outboard Motor

This guide will explain how to change the engine oil and filter of a 15 or 20 HP (Horse Power) 4 Stroke Mercury Outboard Motor. An oil change is usually done after the boating season ends for winterizing or once the engine has run for a certain amount of time as indicated the engine manual. 

Note: Oil changing for winterizing can be done by yourself if you feel comfortable with handling mechanical parts of a machine. However, if you are not comfortable with tools or handling of parts, I suggest you get the oil change done through a marina or a marine technician.

Required Tools

To get the oil and filter change done, you will need the following tools:

  • A Torque Wrench which can reach a torque of 210 lb. in.
  • Socket Wrench and socket set
  • Oil Drain Pan or something to collect used motor oil in
  • Cloths or tissue paper to wipe excess motor oil
  • Oil Filter Plier for changing the oil filter
  • Funnel to fill oil
  • Gloves
  • There is no need for an oil pump. In fact, the oil pump may damage your engine.

What Type of Oil to Use?


If you will be using the boat mostly in warm temperature, 40 degrees Fahrenheit /4 degree Celsius or above, use 25W-40 Engine Oil. 10W-30 is suitable for running the engine in colder temperature, below 40 degrees Fahrenheit /4 degree Celsius. Refer to your engine manual for more information. I usually buy the 25W-40 oil change kit available at most Mercury dealers which includes the oil, filter and gasket. You can also buy alternatives like Quick Silver oil and filter. However, the Mercury kit is more cost effective and includes the necessary items required for the oil change.

Oil and Filter Changing Procedure

Once you have all the required tools and the oil and filter kit, follow the steps below to perform the oil change. The engine oil capacity for a 15 or 20 HP 4 Stroke Mercury Outboard Motor is about 1.0 Liter or (1.1 U.S. quart).

Draining the Used Engine Oil 

Step 1: Tilt your motor all the way up and lock it. Locate the drain plug bolt as pointed by the arrow. When the motor is in full tilt up position, the drain hole will face downward.

motor tilt up
Oil change must be done on dry land. Image shown as an example for tilted up motor.

Step 2: Have a oil drain pan handy and remove the drain plug using a socket wrench and drain the used engine oil. Be sure to dispose of the used engine oil as per your local city regulations. Wear gloves.

Step 3: Once no more oil is draining out, reinstall the drain plug temporarily and lower the motor all the way down. Wait for a few minutes and tilt and lock the motor all the way up again. Remove the drain plug and you will notice that the oil which was trapped in the engine is draining out. Do this 1 or 2 times more to have all the used oil drained out. Wipe out the excess oil around the drain hole using a cloth or paper towel.

Step 4: It is likely that your existing drain plug seal is damaged or worn out. If so, use a new drain plug seal (a new seal comes with the oil change kit from Mercury). Lubricate the seal using new oil and reinstall the drain plug and the seal. Using a torque wrench, tighten as per the torque indicated in your owner’s manual, usually 210 lb. in. It is important that correct torque is used to tighten the bolts otherwise you could over/under tighten and cause damage.

Replacing the Oil Filter

Step 1: Bring the motor down to its fully vertical / upright position or also known as the operating position. 

Step 2: Remove the cover of your motor to locate the engine filter. Put some cloth or paper towels underneath the filter to catch any dripping oil. Then using an oil filter plier to remove the oil filter by turning it counter clockwise or to the left. In some models, you may have to remove the Starter Solenoid to gain access to the filter.


Step 3: After removing the filter, clean the mounting base. 

Step 4: Apply a film of clean oil to the oil filter gasket.


Step 5: Hand tighten the filter gasket back into the mounting base until the gasket contacts the base. Then tighten using an oil filter plier 3/4 to 1 turn. Reinstall any parts which may have been removed to gain access to the filter.

Oil Filling

Step 1: Make sure you still have the motor in the fully upright / vertical position also known as the operating position.

Step 2: Remove the oil cap. 


Step 3: Using a funnel, fill the engine with about 1.0 liter (1.1 U.S. quart) of engine oil.

Step 4: Check the dipstick to ensure that the oil level range is at the midpoint between the operating range.


dipstick operating range

As long as the level is between the low and the high point, you are fine. Do not overfill the engine. 1.0 liter should be sufficient and bring the dipstick level to the midpoint.

Step 5: Start the engine and idle for about 5 minutes. Check for any oil leakage.

Step 6: Stop the engine and check the dip stick again. If there was leakage, add oil if needed.

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