Possible explanations of 10 psi loss of oil pressure at cruise

On my family vacation and on a 5 hour run yesterday I noticed my oil pressure on my stbd main engine had dropped apprx 10 psi at cruise. This was on a VDO gauge at the helm. Wasn’t too concerned thought maybe gauge was messing up but went down in ER to confirm and sure enough the engine room gauge reflected about a 10 psi loss. Now I have gotten concerned.

Boat is a 1995 Viking with 820 hp 10 cylinder Mans. Cruise Rpm 2050 and oil pressure is normally 80 psi and has dropped to 70 psi. These are mechanical engines with no computer. All else appears to be normal. I have visually looked at the oil but that reveals nothing to my eyes. Would rather not deal with a repower right now. But hopefully getting ahead of myself.

Also headed North from Key West. Does anyone have any thoughts on Scott Marine Power in FLL.

Thanks for your time.

Thanks to all who responded.

Used Capt J’s method for checking the oil level and it is midway between the add and full mark. Capt J you were right to suggest this method as I have been down the check, then add, then over full route with these engines.

I would say about 4 hours in the trip is when I noticed the pressure being down. I am a gauge hawk so I feel good about that answer.

Smell test doesn’t smell any different when compared to port engine. Drip test off of dipstick appears the same from one engine to the next.


Don’t have access to my invoices but oil was changed back in the winter with maybe 30 hours on it. Covid prevented use of the boat till now from January. Oil was sampled at that time and nothing abnormal showed up.

Thought I had found the problem. See picture attached of oil pressure sender, but confirmed that ER gauge is mechanical.

Trying to line up an oil change and sample for Monday but Key West is not that easy to find someone on short notice.

Is there a redneck way of seeing if you have fuel in the oil?

Thanks Again

Just tried that and don’t see what you describe and both engines oil looks the same on the napkin. Thank you for the tip.

Capt Ralph and J thanks. You mean go ahead and run the engine as normal if nothing else changes further? Atleast till I get to Miami and see if Scott or that other MAN dealer can get me. Trying to get mechanic down here lined up on short notice is not bearing much fruit. Sure would like a sample and change the oil to see if that changes things. Supposedly there are oil pressure relief valves (I think I am correct on that) on these engines. So hoping nothing catastrophic is brewing.

I realize it has been awhile but I waited and waited for the invoice and oil sample results from the Key West mechanic. Finally got the invoice but no sample. He said lab messed it up. So had another sample done and the oil checked out fine. Negative on fuel in the oil. I have 2 well thought Of mechanics say let it be. Most likely pressure relief valve.

the real point 0f this was twofold. One, I wanted to let the forum know the outcome and two wanted to thank Capt J, Ralph, & M Bevins for their help on this. I sincerely appreciate you alls time and expertise and thank you.