Low oil pressure at idle?

I put a mechanical gauge on temporarily down by filter. Shows about the same as dash. I have changed the filter when I took the Rotella out. Doesn't seem to be making any top end noise with the 5w20 in it. Set my target idle up so it idles around 850, 900. Pressure sits at about 15 to 18 with the idle there. I know the 10 psi for 1000 rpm rule, but I'm used to newly built motors that never run near that low. Just makes me nervous I guess. We have only done a few of these newer motor swaps at the shop. Trying to keep up with the market,and trends and give our customers the options for the newer less maintenance drive trains. Poeple just don't like wrenching like they used too. Just wanna get in ,drive,and feel cool in an old car. We were purely NA here for a long time. I appreciate all the suggestions. Gonna just let her eat for now, and get the 6.1 ready just in case.
BTW I planned on putting the 6.1 in after a rebuild anyway,but this 5.7 pulls pretty hard,and the duster is no slouch for a street car with it.