There could be a few problems
1st drain the oil made put new oil as per manufactures spec as well as new oil filter Test again but let the engine run for awhile?????? if the oil pressure alarm/light stays off then you cold have diesel
fuel contamination smell the old old before you dispose of it is it very thin and does it smell of diesel there could have been a leaking
fuel injector which diluted the oil
2 The problem of
lights coming on could also be ..... Has the engine idling revs been lowered Check manufactures spect it should idle at about 900 rpm with all the axullaries ie feed
pumps refrigeration
pump and alternator on maybe the engine is idling to slow
3 There could be a problem with the sender unit
I had the same problem with my engine a 55 hp Yannma diesel checked all of the above (engine is about 4 yrs old) so I added PROLONG oil additive as a prventive measure it sure helped the oil pressure