Most of the stock and after market manual automotive oil pressure gauge have a very small hole in the inlet side on the back of the gauge, does yours? scope
I've use to use 1/4 I.D. lines to fed my manual oil pressure gauges in my old race cars, they reacted like a tach up scope
NHRA ban them, the larger I.D. lines, a long time ago whiney

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)