I agree with cleaning out the oil jets but if the problem is still there afterwards then don't continue to run the engine but examine the relief valve for being stuck (this is not easy as it is tucked out of the way.)
Presumably the gauge needle returns to zero when the engine is switched off?
I have the same problem. Cleared out jets 3 times. 10psi at 30 in top,20psi in 3rd at 40mph. Big spout of oil if jet plugs in crankcase removed for very short time. Oil getting to timing gears (removed inspection cover on fan pulley housing). 4 jet lube with bored out pump. Need to remove sump to check other 2 jets clear. Same engine mentioned a while ago on here which had water damage and pint of water in sump and 10mm sludge in sump. All carefully cleaned out with new main bearings. Used to be about 10 psi at 40 in 3rd.Close ratio Super Accessories gearbox.