Your first test is to get a mechanical oil pressure gauge and remove the oil pressure sender and put the gauge in its place. Get a true and accurate reading and then you can go forward.
This little cheap one from Harbor Freight is only $24.99 and it will do you just fine.
Engine Oil Pressure Test Kit
You can see the various little fittings that will include the one you need and you just pull the sender and screw the tester in the place of the sender. If you determine that the pressure is actually high you need to find where the obstruction is before you run the engine too much, check the pressure regulator and hopefully that will be it, also change the oil and filter. Let us know if you have any questions or when you discover the issue and fix it, what it took to get it right. My guess is that it is the sender or the gauge.
Side note, you can replace the original Yanmar gauge and sender with a Teleflex sender and gauge. You do have to replace both, because the sender from Yanmar is proprietary and will not talk to other gauges and the gauges will not talk to other senders. If you decide to replace the gauge and sender with the MUCH less expensive Teleflex unit the cost of the two, sender and gauge will be less than the Yanmar sender alone.