Low oil pressure helped by 5w40 oil change

When I bought this truck I didn't know what kind of oil was in it. Maybe it was really thin, or cheap, or ???

It looked dark, as I was expecting, when it was draining, but that's all I paid attention to at the time.

The oil level was always correct when I checked it, and I never needed to top it off.

Otherwise I am not sure why 5w40 and a quart of Lucas oil stabilizer improved the oil pressure It was just an experiment on my part that seems to have worked.

Before the oil change, I was worried, because I thought pressure was sometimes lower than it should be. (worrying about possible oil system problems)

Examples (before the oil change)
After warmup, it mostly showed 20 PSI, and mostly around 30 PSI at 2500-3500 RPM
Once, after driving it for about 40 miles, and then just idling for a short while, it was showing 10 PSI.