Strange oil pressure at depo gauge

[NirAstra]

Hello friends.z22se depo oil pressure gauge,almost 2 years working ok.last week strange reading.....after cold start the gauge vals looks more reasonable,but after the engine go to normal working temperature,the needle jump and never pas 4.5 bar.brfore i used to have 3 bar at idle and extra 1 bar for every extra 1000 rpm .
The idle is as was during the last two years 3 bar +-.
Any ideas? The engine work fine, strong and quiet,oil level as needed of course.bad gauge?
What else can it be?
What are bad relief valve of the oil sump symptoms?

Thanks nir