Never seen this one before. I've got an 08 288 Sunsation with the HO. At about 25 hours or so I noticed low oil pressure on the gauge while running. Around 3000 rpm or more the gauge shows oil pressure between 20 and 25 lbs and the needle flutters rapidly between that range. Not a steady needle at all like is customary. Idling in gear at 800 rpm with warm oil the gauge shows a steady 20 lbs or more. I can bump up the rpm to 1000 to 1200 and the needle immediately jumps up to a steady 40 lbs. Which is what I would expect. Problem is as soon as I start getting up on plane and with the rpms increasing to 3000 or better the gauge actually shows the oil pressure going down and the more rpm the lower the gauge goes to it's lowest point of 20lbs at full throttle but again the needle starts fluttering rapidly between 20 and 25lbs.

This problem didn't start until I got about 25 to 30 hours on the engine. I now have 45 hours. Before then I had great oil pressure (40-50 depending on oil temp) and a steady needle at all times and at any rpm. I changed the oil at 20 hours with the recommended Merc synthetic blend and so far the engine doesn't use a drop. I've checked the connections on the back of the gauge and everything seems to be fine there. I'm hoping maybe a defective sending unit or a defective gauge. I'm also hoping it's not really running that low of oil pressure especially at the higher rpms which would make me think it's an internal problem with the pump. It's just weird how it worked fine for the first 25 hours though. BTW, this happens when the oil is cool or warm. The temp of the oil doesn't seem to affect the results on the gauge much.

Any help or suggestions is much appreciated.

check youre sending unit , ground ect

First thing I would do is hook up a mechanical guage. That will tell you if its an engine issue or just a gauge problem.

I agree with Checkmate. I have a mechanical guage permanantly hooked up to my engine in the engine compartment.

Good idea.
Any thoughts on where a good spot is to tie in the tube fitting on an HO?

Airation or pick up/pump problem?

I checked the engine oil level yesterday while on the water and it was just a hair over full, like a 1/16" or so. I may drain out a half qt or so just to see if it makes any difference. And also install a mechanical gauge to check to the factory gauge.

Drop it to the add line on the stick for a test. You most likely have too much oil and are foaming it! I've seen this DOZENS of times.

Thanks, I'll try that. I'm hoping that is it.
I'm assuming that the "airation" that has been mentioned is caused by too much oil also right or can airation also be caused by something else?

Thanks for all the help guys!

I'm betting its the oil filter. Change the filter and see what happens.