I have an 89 RS 2.8. When the car is started the oil pressure is fine and it stays ok till the car warms up. When I am driving oil pressure stays around 30 PSI, the halfway mark. When I stop at a light it drops down to right above the red, but only if the car is up to temp.

I am thinking the sending unit is bad somehow. Also the car idles really rough when this happens.

 

Put in some thicker oil. When the oil heats up it gets thinner and therefore your pressure drops because it flows easier. You might be a little low too, because I run about 40 psi while driving(warmed up) and a couple notches above the red lin at idle. When the car is cold though, I hit about 60 -70 psi, that gradually goes down as the engine comes up to temp though.

The rough running when warm is probably not related to the oil pressure. Might be that the O2 has gone bad and is now flatlined all the time, making you run lean. You would only notice this after the engine has warmed up because the ECU doesn't listen to the O2 until it has warmed up, the only time that it is giving a consistant signal. At this point the ECU has also gone into closed loop mode and depending VERY heavily on the O2 reading for proper A/F mixture.

No codes, right?

 

Zero codes, as for the oil it has 10W30 in and it isn't low at all.

The needle is erratic also, the whole time.