How to change engine oil in 90 horse four stroke boat motor?

take off the cover and look underneath the engine. you will see a protruding bolt, different to any others as there will only be one and its coming from the sump. loosen it slightly to see if its the right one. Its should be oily. Drain if off and put marine engine oil of the right grade into the filler cap on top of engine. check dipstick if you have one do not overfill.

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