What is ''normal'' DD 71 series oil pressure?

I ran 1978 model 1271N's, they were specd at 50 - 70. Low - 30, dropping lower with wear.

Did the needle bounce when the pressure dropped or just get low and stay there?

Hot oil in a 71 (53, 92,149) can read below 5psi at low idle. Don't get bent.
Sadly, oil flow is measured in this crude PSI measure. If the sump is up, It's flowing.
My stock 5PSI alarms sometimes go off when clutching the engines. All is good.

50-70 PSI is at higher rpm and under load.

This is my experience with most 12 cylinder 71's. Even after majors. I've seen a lot of them after running at cruise, hit the 5 psi alarm when clutching them in and out of gear, waiting for a bridge or something. 8v71's seemed to make more PSI and were around 10-20 psi at dead idle in the same situation for some reason.

Here's a Lube Oil Section for a DD 4-71TI rated 325bhp @ 2600rpm:

When you start the engines, fuel pressure comes up first. A fuel pressure switch around your secondary fuel filter, enables the oil pressure alarms. You only hear the alarm for a second or so.

Also, have you checked your oil sending units?
At about $20 each, they're cheap insurance.
Over 14 years on 1 boat with 1271s, I had to replace the SB unit 3 times. Never had to replace the port unit.

Does your water temp. rise? If the ngine is starving for oil, the temp will rise pretty quickly. It may just go up slightly at first. You have to know where your guages read all the time so you can notice any slight changes.

Wonder if the 28 gal/min stated is @ 5 psi and @ 42/55 psi ????

The 28gpm would be at the rated speed (42-55psi). The flow rate would vary by rpm, don't know what the output is at low idle (5psi).

Find the fuel switch, jump the two wires on it (key switch may need to be on but engine not running), low oil alarm should sound.

another one with two lines coming off of it near the trans upper middle rear of engine..

define lines? Tubes or hoses or wires?

Anything hidden by the exhaust manifold or starter on the block sides?

Re your secondary fuel filter, is there a sender around it? or a small tube/hose going to a block and one or two senders on this?

J/T luvs to hang lots of gizmos (Junk&Trash) on their engines.

The oil pressure sending unit is electrical.
On DD, it's about 1 1/2" diameter, screws into block, has a single threaded post with nut to attatch wire.
I'm not sure but I think newer engines could use a different 2 wire unit.

Dang, can't load a picture!

It's located on the right side of the block. It simply screws into the block and the wire attatches to the other end. Believe me, when you see one, you can't get anything wrong! It's very basic. Screw in - attatch wire.

Only thing to remember! Never use Teflon tape or thread sealer! The unit threads must contact the block. One wire, remember?

Looking at other responses though, I don't believe the sending unit is a problem but you should keep one on board for a spare.

Yep!
Jeeze, they didn't give you much room!

Yep, little one for the alarm, big one for a gauge.