Mine takes about 4 to 5 seconds to wake-up after a cold start. Than it jumps to 3 bars. when hot, at idle, it goes down just below .5bars. At 1000RPM and more, it's always back to 3 bars, hot or cold.
Been doing that for 11 years and 235K.
Could be a sluggish oil pressure sending unit, a lazzy gauge or anything in between.
I would not loose sleep over it if I was you. (But I'm not)