When looking through the standard owner’s manual for a boat engine, it’s important to note that the engine oil capacities listed are approximate. In addition, the engine oil capacity is reflective of the entire lubrication system. For this reason, when filling the engine, sometimes boat owners will notice that the engine is filled before the indicated amount of oil is used. Once oil is inside the engine, it’s quite difficult to ensure that all the oil is drained during an oil change. The residual oil left in the engine is the reason that it cannot be filled with the recommended amount of oil.
For a popular engine model, like the MerCruiser from Mercury, it’s important to fill the crankcase according to the dipstick when the engine is level. If the oil is at the mark, then it as the proper level regardless of how much additional oil is put in. Never overfill the crankcase, and always ensure that the right oil is used. For example, for a MerCruiser engine, use Mercury oil from the manufacturer for better performance and to preserve the life of the engine. The engine oil level should be between the add and full marks on the dipstick.