It seems (I could be wrong) that a painted/dipped surface coating wouldn't need oil to protect the finish/metal like bluing or parkerizing does.
But I still wipe mine down with an oily cloth (yeah, a rag) when ever I clean it or put it away in the box for some reason. And, on some guns, holster wear has worn completely through the polycoat showing bare metal and that will rust if not oiled/wiped.
I don't really recall a finish looking spotty after wiping it down with that oily cloth, but I don't wipe off the excess either. I just wipe it down good and put it away.