So I recently purchased this boat and realized shortly after that it had low oil pressure. It would be @ 40psi running and nearly nothing at idle. So I replaced it with a automotive one of the same PSI rating and now its reading 80 PSI running and pegged at idle. I'm unsure what could be going wrong does anyone have any suggestions? Think I may just bite the bullet and pay big bucks for the Mercruiser sending unit!

These motors run low oil pressure when that oil gets hot....no coolers.

10-15 psi at idle after a good run is not uncommon on these.

Sending units are rated by the ohms they put out, not by the psi. The ohm output moves the needle a certain amount.

I just was doing some searching on the internet and these engines do have low oil pressure at idle, Seems very common! So where can I add a oil cooler if this engine is not equipped with one?? I usually am running around 3500 rpm all the time so I'm not running too hard. And I'm going to put the old oil pressure sending unit back in since its likely ok.

How easy would it be to add a cooler.... after market or merc??

The blue scorpion motors had coolers.

Do some reading and ideally get an oil cooler and an oil thermostat. The stat allows the oil to pass by until it gets to the desired temperature then opens allowing it to run through the heat exchanger. Cold oil isn't that good either.
I built a medium performance 383 last year with normal bearing clearances. I get 15 psi hot idle and 55 or 60 psi hot at anything over 1100 rpm. Has 30 hours on it now.

The coolers aren't hard to rig. Incoming water goes through one set of openings, engine oil through the others.