The problem you are having is not entirely unheard of on this engine. Normally it can be solved
by a good grade of 15/40 or 20/50 oil like Rotella T, or sometimes just a few moments of higher RPM
without load to cool down the oil. It could, of course, be something serious as described by Lepke. In any event there are a few things that you can check before diving
into the engine.
First thing is to actually check the oil pressure and see what you have. The oil pressure specification for the D1-30 at idle, hot engine, is "greater than 7 psi".
To check this, remove the oil pressure switch on the Stb. side of the engine and install an accurate gauge. The gauge shouldn't have a range greater than maybe 20 psi to ensure that you get an accurate reading. Replicate the conditions that cause the alarm
and see what pressure you get. If the pressure is around 7 psi, you should check the alarm switch. The switch is normally open when oil pressure is present and should close at .5 bar (7.25psi) plus/minus .15 bar on falling pressure. That means it should make contact between 5 psi and 9.4 psi on its way down. If you check it on rising pressure it won't be as accurate.
Early on with this engine the switch value was too high and many false alarms occurred until the switch value was lowered. If you get a replacement switch, check it out before installing.
If the oil pressure is actually low, the next thing to check is the oil pressure relief valve. Pressure relief valves are designed to protect the oil pump and oil system from too much pressure, so you can wouldn't think they would cause a low pressure problem, but sometimes the spring or plunger in the valve can get hung up and allow a small leak back into the crankcase. This shows up as low pressure when the engine oil is hot and the engine is idling. Luckily, on this engine the relief valve can be accessed from outside the engine. It is located just adjacent to where the oil switch is located on the Stb. side of the block, and it has a hex head
that looks like a large bolt screwed into the block. Remove the valve and check for dirt, debris, etc holding the valve open a little bit. There will be a bit of oil leak
out when you remove the relief valve.
If everything looks OK with the relief valve, the oil pump is next. The oil pump is located behind the front timing case on the front end of the engine block. To get to it you have to remove the front crankshaft pulley, the seawater pump and then the timing case itself. Before the timing case can be pulled away from the block, the injection pump must be loosened up a bit so that the governor linkage can be disconnected from the inj. pump. The oil pump is a trachoidal type pump with an inner and outer rotor, and a shimmed front plate. I doubt there is enough wear in the rotors to cause any problem at these low hours, but sometimes the front cover isn't shimmed tight enough and needs to be reshimmed to recover some lost
pressure. This is easily done with the available shims, and the clearance should be set to the minimum spec.
The last thing that might be an issue is the O ring fitted to the oil pump suction tube, where the tube enters the engine block. Any damage to this O ring can cause pressure issues. Access to the O ring entails dropping the oil pan, and if you are that far in you could also check the rod bearings for excessive wear, although I think you might hear it knocking if this was a problem.
Some of these measures might require the services of a trained technician, but if you want to get an idea of what is involved, Google
" D1-30 service
manual" and you will find a couple of on-line sites that offer the manual for free. I think the Manta
owners site is one, and also the J109 site. (j109.org)
Hope this helps some....