In my merc 420, I get no more than 40psi ever. When it's cold, I get 20psi at idle and 40psi wot. Hot after a long run its about 18psi idle and 38psi wot. Is that ok, or should I put in a high pressure pump. Motor has 139hrs and runs well.

I would check it with a mechanical pressure gauge at the engine. If the one on the dash is accurate then NO you don't have enough oil pressure. If this is a stock engine then something is wrong with the engine And I would fix that problem instead of a high volume pump.

not enough if its true reading, you have issues and its gonna spin a bearing very soon, check with Mech gauge and cut oil filter open and see whats up in there..

I was reading Merc service manuals and its within spec, but i hear of guys running 60-80psi so I was wondering. I was hoping the motor was setup a little loose with a stock volume/psi pump. What should I look for?

Don't worry about what others are running. If it has been running those pressures for 139hrs and survived you are just fine. If last month it was 80psi, last week 60psi and now 40psi I would then be somewhat concerned. Verify with mechanical gauge and also change the oil and cut open the filter and investigate. Also what weight oil are you running? That affects pressures. My engine oil pressures change quite a bit from Mobil1 15-50 and 20-50 VTwin.

I dont spin it to 5k ever either. That 40psi is at 4000-4200. I never really wind it out. I am running Mobil 1 15-50.

I should have said higher RPM not WOT.

depending on engine setup 10 psi per 1000 rpm is idea of close, I don't like that rule personally and I run way more pressures, just a safety zone as far as I am concerned... why be on the low side of acceptable? loose clearances and high pressure works for me... too tight a low pressure will hurt ya, but yes it has many hours and always been that way I doubt there is an issue, but if it had more and is dropping its not going to be good...

I just got it in the winter at 92 hours. The oil pan leaked like crazy where the dipstick goes in. So over winter I did all new intake, head, valve cover, oil pan gaskets with the new pan. I didnt want any oil leaks. I looked inside while it was apart. I saw zero metal particles. I also see zero shimmer when I change oil. It now has 140 hours. The pressure has been 100% consistent during my ownership.

If this has the old school VDO gauges on it, put a mechanical gauge on it and verify pressure. On my Formula I pretty much only replied on them to give me a suggestion on if I had oil pressure or not. I put mechanical gauges on both my motors, verified I was getting 50-60 psi at WOT, and stopped worrying about them after that.