Mercruiser oil pressure problem ?

I'm going to look at a boat today. The owner says the engine has low oil pressure 5 psi. Its a 3.0 mercruiser I/O. They say it runs and sounds good. They never had a mechanic look into it. It had higher pressure but just dropped to 5 psi at some time. A mechanic told them to just use heavier oil and run it. So they did. It sounds like an indication problem to me. Has anyone seen this before?