The motor oil I use in my Mercruiser 454 MAG MPI Horizon engines is the Mercury Marine 25W-50 synthetic blend 4-stroke marine engine oil. I do not use additives.
On my boat I designed and installed a custom 5-zone high-volume high-pressure oil change system that will suck out the oil sump in ten seconds. It comes out like a fire hose. This is part of my 5 minute oil and filter change system, enabling me to keep fresh engine oil for long life.
Mine is stored and operated in salt water.
IMO the big engine issue is the raw water (salt water) cooled exhaust. This causes exhaust manifold, riser and elbow corrosion damage, creating the possibility of raw water intrusion into the engine.
Before I redesigned my custom exhaust cooling I was getting about three years on genuine Mercruiser parts before I was unwilling to continue usage. I would perform an annual inspection of the solid wall thickness between raw water passages and exhaust gas passages.
Now my exhaust is rinsed w/ fresh water, soapy water then Salt Away. When done I drain my exhaust bone dry until the next usage. It looks like my expensive exhausts will now have a very long lifetime.
