Oil pressure light at higher speeds - Eagle

I know the easy answer is "don't drive on the highway", but any thoughts?  It's an 81 Eagle with the 258 and the auto transmission.  The oil level is fine, engine temp is fine, and there are no sounds out of the ordinary.  

Your situation seems very similar to looking at the oil pressure bypass and more specifically the bypass spring.

It could be weak over time and cycling.

The other possibility is bearing wear , but unlikely if the light goes off at temperature and just lowering speed.

You have not mentioned mileage, oil burn, recent oil grade change, filter change.

A bad filter or lowering grade oil can cause this.

The pump at idle doesn't move enough fluid to move that valve as it can't build 70 psi - there's not enough volume against the restriction - it moves through the engine and builds less than 70 psi.
I'm sure many have experienced a car with power steering and there's a bit of a problem with the idle situation and the engine speed isn't sufficient to spin the pump to put out enough VOLUME so it doesn't build PRESSURE and you find you can't turn the wheel or it's sluggish or jerky. That's a pump not putting out volume so it doesn't build the pressure needed to aid in steering.
Back to the oil pump.....
So at idle, you want that relief valve closed so the fluid isn't let out as if it was it would be like a leaking hose and oil would never get to the end - it would dump out the relief valve and not get to the last bearings, etc.
If the relief valve is stuck open, at lower speeds, lower RPM, you lose enough volume through the stuck relief valve you have LOW pressure at idle.
But rev it up and the pump can put out enough VOLUME that it can actually build pressure - you now have more volume, fill the pipes and build pressure. 
If the valve was stuck very open, you'd have no pressure at idle because the pump can't put out enough volume to fill the system and build pressure - volume against resistance, there's no resistance.
If it was stuck a little open, you'd have lower pressure all around but a fast spinning pump may produce enough volume that you are still ok at higher speeds.

My point is that a stuck open relief valve will impact you at idle speeds, but one the highway, you spin the pump fast enough to still have pressure.
Volume builds pressure - try it - rig the valve so that it's partially open all the time, you'll see no or really low pressure at idle but spin the thing up and there's enough oil being pumped that some can leak past the valve and still have pressure - maybe LOWER pressure, but above the 8 to 10 psi needed for the light. At idle, however, you can't move the volume needed because it's bleeding out past the partially open valve. 

It's exactly like an engine with WORN BEARINGS - you see the light come on at stop lights, but rev it up and the pump can overcome the leaking bearings. The relief valve if stuck open is the same as bearings that are worn - it would let oil bleed out and not let pressure build because the pump at idle can't keep up, but rev it and it has enough volume to build pressure and shut the light out. 

I'd do what farna suggests, and it's a cheap test-  CHANGE the filter. Buy a good new filter and swap that on. We've actually seen filters come apart inside.
If the filter were plugged, the pump may push enough oil to keep the system filled at lower speeds but maybe enough oil can't get through the filter to satisfy an engine at higher rpm because there's more oil leaking past bearings, lifters, etc.

I can see how a filter could in theory do this, but a stuck relief valve should do the opposite - lose pressure at idle.
Again, also use a mechanical gauge to know for sure.........