GM-GMC: oil presure, mobil delvac 1, oil pressure gauge

Question
I have a 92 c3500 with 6.5 turbo.  when I start engine cold I have 45psi oil presure when engine warms up to operating temp oil psi drops to 5 to 10 psi between idle and 2000 rpm. Ihave changed mains and oilpump still no change.  Oil cooler lines are free and clear of debreis. Ihave a mechanical oil presure gauge hooked up.  could oil cooler bypass cause this problem? thanks

Answer
Where are you measuring the oil pressure?  I have the OEM electric oil pressure gauge installed at the back of the block and a mechanical oil pressure gauge installed at the front of the block in my 1983 G-20 6.2 diesel.

The mechanical gauge always reads lower than the electric gauge, as it should since it reads pressure near the #1 cam bearing, and I have seen it at 15-20 psi at hot idle.  With a previous 6.5 engine and Mobil Delvac 1, I saw readings below 10 psi on the mechanical gauge at idle.

You might have a sticking (or weak spring) in the oil filter bypass valve.  The oil cooler should not be causing your problem and you should not disconnect it.  You could also have worn cam bearings and they receive oil before the main bearings.

What weight oil do you use?  You might try a straight weight, such as SAE 30 that GM specified for use in the early 6.2s at temperatures above freezing.