Sorry I should have clarified. I don't have an aftermarket oil pressure gauge, I just have white faced dials in my car and they read in psi instead of kg/cm.
But yeah the factory oil pressure gauge is a bit suss so I take it with a grain of salt.
As far as I am aware, there is nothing that simply plugs in into our cars, you will always have to wire something in. And besides, it isn't so much the gauge itself that is crap, it's the 15+ year old senders.
At idle mine sits on like 25-30psi these days. Sometimes it falls towards zero and scares the shit out of me, but it is just the factory gauge acting up.