{I'm asking a question as a hypothetical owner:}
I have a 1989 Ford Tbird with a 1990 Merc Cougar engine 3.8.
And since I changed the engine, the oil pressure is low..
It is higher on idle than with the ign turned on but not running, but on the road does not ever get to the "Normal Range" though it does go up
I just changed the oil pressure switch not the prob still low oil pressure
Answer:
Since you changed the engine, I suspect the problem MAY be in the sender used. Some senders are switches, some are pressure sensing variable resistors.
The Cougar being a slightly higher range car may use the variable type which are more expensive and larger in size, about 1 3/4 to 2 inches in diameter. Switches are about half that diameter
TBirds and Mustng LX around that time used a switch and resistor in the gauge circuit so that when there was oil pressure at idle or speed it would always be in the same place on the gauge... approx mid-normal.
Check the gauge by grounding the lead that connects to sender against engine with key on. If it goes to midrange of the word "Normal" it has a resistor in the gauge circuit, and expects to see a switch - either ground or nothing.
If it goes to top of gauge it doesnt have resistor and a "low" reading IS low... Whether you have a switch or sender.. So then the problem may be in the oil pump itself
In 93 or so, the resistr and is on the back of the gauge cluster and if you have the variable sender you must solder a wire from one side of that resistor to the other to get normal readings with the variable sender.
If your pressure varies from very low (but higher than ign on) at idle to just low at speed, then you PROBABLY have a variable sender so check for that 20 ohm resistor by grounding the sender lead.
I hear on others it may be in the lead at the sender end.