The 300DT is equipped with an oil cooler. When the oil temperature is below the set point, there is a by pass to enable the oil to warm quickly. When the oil reaches the set temperature, a valve open and the flow (or a part of the flow) is sent to the oil radiator.
My guess is maybe when the regulation valve open and close, it disturbs the oil pressure and it generates some fluttering effect on the needle.
It's just a hypothesis.

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in France : 240D 1981 380 000 km
240D 1984 252 000 km
300D 1978 325 000 km
in the US : Ford Tempo 1993 70K