Not sure if it applies to the Mercury 4 strokes. I know from my drag racing days before we went to a dry sump system that if oil level was ran at full on normal wet sump system dip stick. Oil pressure would actually drop at around 6000 - 7000 rpm. Running 1 - 2 quarts low would result in consistent oil pressure. Too much oil in engine was causing aeration of oil and premature rod bearing wear. A good reason to follow Don's advice on only fill to half way on dip stick. These engines run at a relatively high RPM. Changed my oil this week 5.7 quarts put level just before 3rd ball.