Greetings,

Why don't we try something, although most think it's unheard of. Let's measure the oil pressure using another reliable gauge connected to the gauge port at the oil filter housing. If you get the same results after running the engine for a very short interval, look to the oil pump as not meeting it's pressure requirements. Can't say anyone here on this site has heard of oil pressure just dropping out of sight overnight and it's the engine gone bad, unless you've run it dry on oil for a day. If this isn't the case, it's the oil pump gone bad or the check valve in the system giving you a false reading on the gauge. Start the engine with the oil fill cap off and see if the cam gets fed with oil to the cam oiler under pressure. If so the engine is getting oil but either the filter bypass is full open and stuck there or the pump check valve has gone bad. Check it out before replacing a good engine with the same.

Charles