Had a similar experience the first time I drained the torque converter. Refilling took much longer than I anticipated - add a bit, check, add a bit more, recheck. Half of it was I didn't know what to expect and thought I was doing it wrong.

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1983 300SD 467,000 km - 473 champagnermetallic

location: 34°5' N 106°53' W (Socorro NM, USA)