I finally got around to adding an external oil cooler on the 440 Dart.
I heard many good things about the oil cooler from a mazda RX and thats the oil cooler I installed. This cooler is expensive new but I always feel you get what you pay for.
This is a welded tank cooler healthy flat tubes in cooler with a built in thermostat to bypass the cooler if the oil temperature is less 160F. The fittings are 18MM adapted to JIC 10 I used a doughnut adapter in front of the filter and 5/8" ID hose to JIC 10 fittings. Lines are about 16" long and are not compressed especially at 50 PSI.
I thought the setup would have very low restriction on the 440 oil system.
BUT the oil pressure was always 65 PSI when normally hot running now the pressure has dropped to 50 PSI under all conditions when the oil thermostat is open.
I am thinking there could be a pressure regulator in addition to the thermo bypass but also could be restriction in the system.
A 440 needs all the pressure it can get and I am concerned about the 50 PSI especially when I run it up past 5K.
Any ideas?

I think the 1st move would be to as you said open it up as something is malfunctioning to be effecting almost a 25% drop

I would say that the cooler has a greater restriction
than you think it does... try jumping a hose around
the cooler to see if your pressure comes back

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...but also could be restriction in the system.

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Wouldn't a restriction in the system cause an increase in pressure?

Are you referring to an oil cooler for a Mazda RX-7 wankle rotory turbo engine? If so that’s different animal than an old school 4-Stroke 440 Chrysler engine. Is 160(F) the stock oil temp setting by Mazda engineers? That’s interesting if it is.

Really, motor oil operating temperature is best optimized at around 212 degrees (F) for a typical water cooled 4-Stroke engine. This will actually free up a few extra ponies versus 160 (F). The design oil temperature for modern water cooled engines is actually 212(F). It’s also why the pros want HOT oil when dyno testing; thinner, more flow, more horsepower, etc.

Unless one regularly sees oil temperatures over 212(F) such as racing and or turbo applications, it’s probably more detrimental than beneficial to utilize an oil cooler. This is where paying close attention to an accurate oil temperature gauge pays off. The gauge could be telling you that you need an oil cooler, or it could be telling you that you don’t need an oil cooler.

What’s your oil temperature gauge actually telling you?

IMHO,the large 10jic line is the pressure drop.

need a HV pump to pump more volume to raise psi.

I would reduce the line size to a 6jic max

hope it has extra pan capacity for higher rpm