That's just how the gauge works. That factory gauge is really just good for knowing when you're at extreme low pressures or an extreme high ones. Pretty much useless in determining oil levels. Need a mechanical gauge for that. Although... looking at the dipstick would work, too...

(You know an engine is low on oil on a mechanical gauge when the needle radically bobs up and down while accelerating or decelerating. My car has a mechanical... factory gauge on it is a dummy light, with a busted bulb.)

Anyway 20-40 is normal. Also oil thins out as it gets to temperature, so that would be the reason why you note different pressure at idle when you first start your truck up, compared to after it's been running for a while. On the flip side, oil can get real thick when it's too cold out, making it run at even higher pressures from cold start. Put the wrong grade of oil in for the wrong climate, you can run into some problems, I guess. Like say... in Alaska, maybe?