When changing over to Mobil1 ATF, should you first flush the system, or can you just start topping off with it?

i would recommend removing whatever old fluid is left in the reservoir,then topup.then repeat the process 1 more time after youve let the car run for a spell

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michael cole 1980 300SD gone now but not forgotten,87tdt,90 300te 4matic 95 e320 wagon