Iveco 75e15?

Help please; 1996 Iveco 75e15 6 cyl turbo; I am getting low oil pressure when the engine is hot, is fine for about 30 minutes but then oil pressure drops, and when I'm idling it is below the red , originally thought I had fuel in the oil, as I had a fuel problem (possibly with the lift pump), Just changed the lift pump, and took the sump off, no swarf in the sump, except some bits of what look like gasket, changed the oil (15/40) and filters, but the problem persisits. The engine has only done 51000 km, even though it is 22 years old. Do we think its the oil pump? Or gauge? Someone said it could be the bottom end bearings, but the engine sounds fine, and I saw no evidence of bad bearings, but didn't take the bearing carriers off as I don't have the torque settings to put them back. I'm at a loss.
Any help greatly appreciated.