Why the Oil press level decrease to minimum after the less one minute from startup AC unit and has been steady state on minimum level ?

The oil in a refrigeration system should remain as clean as it iswhen it enters the compressor which is quickly contaminated by fuel, water carbon and atmospheric dust). The condition of the compressor oil is therefore a direct indication of the physical and chemical cleanliness of the system Overheating or an electrical fault in the winding of a hermetic orsemihermetic compressor motor will produce contaminants,including the halogen acids.. All those  reason above it has been made abearin a wear out  on a crank shaft and bearings and the oil became axtra bleeding oil from bearings, so the oil  presser decrease to the  low level and the second reason oil filter may be blocked.