check lift pump pressure under load, but I can almost guarantee you have a bad OPS not letting the lift pump run. 95 and older is extrmely prone to this.
Newer OPS's are not as robust as the original, so you will need a relay harness in addition to a new OPS.
www.leroydiesel.com sells a good one, and his has the WTS prime feature like a 96+, very helpful to overcome a tiny air leak.
Before you do final checkout with Leroy, check the condition of the harmonic balancer and the damper pulley. Leroy sells improvements for those, as well as oil cooler lines.
I am a glutton for punishment I guess, but I am always worried about a cornstalk snagging a cooler line, and so on the 96 and older low output oil pump systems, I stick with GM lines, they tuck out of the way better. They are leak prone though at the crimps.
If you dont have to worry about off road obstacles snagging them, Leroy's lines are definitely superior.
Basically the preventive maintenance is the same as a gasser, fix leaks, inspect the harmonic balancer, make sure your fluids and filters are changed often, thermostat is good, etc.
Reputable vendors are
www.leroydiesel.com and
www.quadstartuning.com
What is your usage for this vehicle? If you dont plan on extreme loads, I'd say a lift pump relay harness and a factory airbox from a 1997+ 6.5L is all Id do outside of maintenance. These things are actually kinda spunky when caught up on maintenance, guys love comparing new upgraded parts to old failed/ wore parts and then crow that upgrades are mandatory to pull a friggin car trailer. Then they have 10K+ wrapped up in it and then get stomped by a HPCR diesel then get mad. lol
I pull a loaded 30' gooseneck with a stock 95 6.5L NV4500 and get by fine. yes, im in 3rd on most grades, 2nd on the nastiest ones, but it hasnt failed to get me over the mountain, and she'll roll 65-70 on the flat just fine.
So just to recap, buy a new AC Delco OPS, and a leroy diesel lift pump relay harness. start there, and get back to us on how it performs after that.
