... I’m finding with longer motoring and engine run times, my oil pressure gauge sometimes slowly drifts down towards to Zero. If I ever-so-slightly increase or decrease the engine RPM’s, the gauge immediately goes to normal and will stay there for awhile, no matter what the RPM’s are. I only notice this problem when the engine has been running for well over an hour.

The engine oil level is right at the top line on the dipstick. My filter & oil were changed 50 hours ago, although that was nearly a year ago. This is a well-maintained 25 year old Yanmar 4JHE with 4000 hours.

Any ideas … clogged oil filter; dirty or clogged engine oil pressure sensor; faulty oil pump ??

Thanks, cheers, John