Oil Level & Pressure

[lalijaca]

Took the boat out today for the first run of the season and test out my shoulder after having severely dislocated it in June playing Aussie Rules. All went well, except for a question I have on oil level and pressure.

When I changed the oil in the engine (MP 340 HP), I slightly overfilled it. I was suppose to add 5-1/2 quarts (5.2 litres) as per the forum recommendation and its closer to 5-1/2 litres (5.8 quarts).
I have checked the dip stick and it probably 10mm (3/8")higher than the fill mark.
I figured 300ml was not going to hurt but I noticed that the oil pressure was up there, higher than normal and at skiing speed pegged on the highest blue line on the gauges. When I went faster, the oil pressure actually decreased.
Why is this and should I remove the extra 300ml.

[Ewok]

If you overfill oil, it can cause engine drag by making it harder for the bottom end of the Pistons to move. It can also cause the oil to get frothy from being agitated more than designed, which might cause the oil pump to cavitate and drop oil pressure. That is why you shouldn't overfill oil.

I don't know if your little bit of overfill is enough to make that much trouble, but it can't hurt to drop the oil level to the recommended level on the dip stick.

[ericinmich]

Yes, overfilling can get the level high enough that the bottom of the crankshaft (rod bearings) to whip the oil. Foamy oil won't pump well or lubricate as well. As said, drop the oil level to normal (suck some out). I bet 3/8" is more than 0.3 quarts as you suggest. Some probably was left in when you drained it (or sucked it out).

[IDBoating]

Standard rule of thumb is "Better to slightly underfill the oil than overfill it". Definitely drop that level to no higher than the full line on the dipstick, preferably measured when the boat is level in the water. Then run it again and report back.

[lalijaca]

Well that was very interesting.
I promptly removed between 550-600ml of oil to make the oil level with the full mark of the dip stick.
I will check oil pressure again on the weekend.

Note, Will the oil be OK, or should I change?

[ericinmich]

Oil will be fine. You just had about a 90% oil change which most end up with anyway. BTW, you are posting in ml just to make us 'mericans have to think I bet!

Quick summary on oil. It's made up of hydrocarbon molecules , some long, some short, some in between. As oil wears (based on each revolution) the oil between rotating surfaces does it's job by not allowing the metal to rub on metal. At a microscopic level, the longer molecules will sheer or break in pieces and over time, there is less protection with the short molecules vs the long ones. When you left 10% of the oil in, it will only matter if you push your oil change too far. If we change the oil as recommended by the manufacture there is a large safety margin. I worked on oil life algorithms at an OEM, there is a bunch more science in it that what I summarized... but this is the most important part....