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If your boat has a remote oil filter, it will be mounted in a similiar location to the one in the picture below on the top port side of the engine (black oil filter with red stripe that reads Mercury, your filter may be white in color as both filters are offered for your engine)...
If you do not have a remote oil filter system installed on your boat... below is a reference picture of a GM V-8 engine that shows the oil filter location on the engine block (blue filter on the bottom right corner)....
If you're facing the front of the engine, the filter that you are describing on the left side of the engine (starboard side) will be the water/fuel seperating filter (looks identical to an oil filter).
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Your situation is what all marine technicians have to deal with every day, the boat builders rarely give us enough room to do our jobs.
Since you dont have a remote oil filter system, it can be a real bear to get to the oil filter depending on your engine compartment configuration. If the engine sits deep in the engine compartment it's even more difficult. I will often lay on top of the engine if needed and go in upside down to access the filter if there is no room on the port side of the engine.
I normally use a flexible oil filter wrench (the type with the handle that swivels from side to side) to help remove the filter as you can move the handle around any objects in the way as you loosen the filter. Be sure to put several towels down in the engine compartment below the oil filter when changing it (helps catch any oil spilling out of the filter). Also, you must BE SURE that the old oil filter's seal did not come off and stick to the engine block before installing a new filter otherwise you will have a major oil leak (also be sure to coat the new oil filters seal with oil before installing it). Tighten the filter securely by hand (do not use the filter wrench to tighten the new filter or it may seize onto the engine block and will be a real chore to remove next time). Once you have the new filter installed and engine filled with oil, test run and inspect for any oil leaks and check your oil pressure to make sure you have good oil pressure.
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