RD28ETi with High Oil Pressure

[oldtimer40]

Hello Ladies & Gents,

Used to have a 1996 GQ 4.2 Petrol Wagon, now I have a 1998 GU Wagon with RD28Eti Diesel engine. Has been going great for the last 2 years until one day driving down the highway the other day, I thought it sounded loud and droning, so I decided to install an oil pressure gauge, and the oil pressure if way off spec?? I brought a shop oil pressure gauge to check the reading and it's not the gauge.

On a warmed up engine, I am getting about 30PSI (Manual says it should be more that 11PSI) and at 2500-3000RPM I am getting 70-80PSI (should be 40-60). So I decided to run a engine flush treatment through it and change the oil, thinking it would clear any obstruction. I am using Penrite HPR15 15W50. While I had the oil filter off I looked at the ball-bearing valve, and it "appears" to be working ok - no cracks, or movement and spring working.

There seems to be an obstruction somewhere, but manual says to check the oil pressure relief valve? Is this the ball-bearing valve behind the oil filter, or is it somewhere else?? What other areas do I need to be looking at the find the obstuction? Any Ideas?