Outer Limits 4x4 Board

Hi all, I have a ? for you guys. The oil pressure gauge on My 75 ute(1hz motor) vareis quite a bit, I have checked the owners manual and it shows that the gauge should indicate aywere between the two middle marks of the gauge during normal opperation and, anywere from a bit above the low mark to the 1st mark during idle.
The oil pressure that is being indicated now when under normal opperation is slightly above the first mark(about horizontal) to just below the first mark and, when at idle it indicates the lower idle point of the gauge.
I am wondering if the oil i am using(Castrol GTX Diesel) is causing this drop in indicated oil pressure. Befor I did the first oil change from when I bought the truck the pressure didnt seem to be as low as indicated. I have also changed back to genuine totota filters as I did try Ryco but it hasnt made any difference. The truck has had all services done at Totota up to 120 000km and now has 131 000km on it.

Has anyone else come across this happening with different oils, filters?
Is there possibly a problem with oil pump?

Look forward to your responces

common all but one of my mates 1hz including mine run low oil presure
and dont run rico filters and always use the quality oil use a thiker one if you think the presure is to low but mine now has done 15000 like it with some heavy long distance towing

use Penrite HPR diesel and the oil pressure WILL come up.

put a guage on it by unscrewing the oil pressure sender out of the block. if thats ok then its your guage and ive had a quote on mine and it wasent cheap. I got one with 380 000 ks on it and oil pressure is good but a little better with HPR 20/60 oil in it and donaldson filters.