The indicator in question is an oil pressure gauge. Oil pressure is naturally lower after the engine warms up due to the fact that the oil gets thinner when it gets hot, this is why there is lower pressure after the car is warmed up.
The gauge is callibrated in bars. What weight of oil do you use? A thicker oil will result in higher pressure. The gauge is callibrated in bars of pressure, as long as it reads at least 1 bar at idle only and pegs at 3 the moment engine RPMs are brought up, then the oil pressure should be within the acceptable limits.
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