It is very rare that the relief valve is out of adjustment, or that tightening it up will raise idle oil pressure.
Unless the spring is broken, the plunger is stuck, or something is holding the relief valve open, the problem is not in the relief valve.
Is this something new? As in a sudden loss of oil pressure? Is there any thing else unusual, like temperature increase, loss of power, unusual noises, extra blow-by?
If any of these are present, there is a problem that needs addressing.
But if it's been running near that pressure, or has very slowly dropped over the years, and everything else is still sounding and running the same, I would not be too concerned.
General rule of thumb, minimum allowable safe pressure is 10 PSI per 1000 RPM. That means if it has as low as 5 PSI at warm idle, it is moving oil through the system. I've seen old engines that would barely move the needle at idle, and they ran that way for ever.
Also consider it's use. This is not likely a working truck, and I hope it's not getting out on the freeway! So it probably has a fairly easy life.
Other things to consider, any chance the oil is contaminated with gas?
Has anything else been changed, like a hotter thermostat (if equipped)?
Does it have a canister type oil filter, and was it recently changed or added, or modified? The filter is likely a by-pass type that meters a small amount of oil through the filter and dumps back to the crankcase. If the orifice was enlarged or removed the oil pressure will drop.
The old engines were designed for straight weight, usually 30 weight oil. I personally prefer 15w40 diesel oil in the older engines, or stay with straight 30 or what ever the mfg recommended. It's rare low oil pressure can be corrected with heavier weight oil, but very light oil can lower it.
Hope there is nothing serious and it will be around for many more years!
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