What could be potential causes (and implications) of high oil pressure readings? Say, the gauge stays somewhere in between 80 to 90 range. What are most common symptoms?

(Checking out another xjs to compliment my "sick" one.)

 

Broken gauge... Those oil pressure gauges are as reliable as a throwing stones to determin the temperature...

 

Another cause might be stuck-shut oil pressure relief valve. If the reading is accurate, as Daim mentioned. It could be much worse (eg blocked feed pipes after the gauge sensor) but a bit unlikely. The gauge reads off the supply to the cams, so undo the oil filler, let the engine tick over and look to see if there is oil about.
Greg

 

faulty wire to sensor... as the heat under the hood likes to break down the insulation on the wires...
unhook the wire to the sensor... if no change ... look to wire...
if change look at sensor... hook up a manual gauge and check real world pressure...
I have to replace my coil left bank low voltage wire as the insulation has all fallen off... just bear copper... sweet!

 

Maybe a bad ground or corrosion on the "wiring" behind the instrument panel. If you've never had the panel out, it's an eye opening experience to see how poorly the "wires", actually just copper channels in pressed on some short of flexible plastic sheeting, make contact. Cleaning up the contacts may be all that's needed.

Might want to examine the top electrical connection on the sensor. If it looks like oil is seeping out of it, then oil has gotten where it shouldn't be (inside the sensor), and it may be just a matter of replacing it.

John
1987 Xj-S V12

 

If I'm not mistaken high resistance equals low pressure reading. Low resistance equals high oil pressure reading. Easy to check. Ground the oil sending unit connector. Low resistance. Should read off the top of the gauge. If so, you may have a short in the wire that's grounding or partially grounding.