1991 300e low oil pressure

Have a 1991 300e with 140,000 km. The oil pressure gauge drops right to the bottom on idle. The engine runs fine lots of power. There is no
oil in the water and no water in the oil. Could the oil pump need to be replaced at this low mileage or could it be the sending unit. any suggestions. I have later model 1993 300 e with higher mileage and it reads in the centre at idle.
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