I drain it whenever the mileage indicates and do it hot.

I do not consider it of any consequence on my Merc diesels whether there be a 1/2 quart or so left upside the oilpan as I use synthetics.

On my antiques,however,especially the 30-98 I drain it hot,immediately fill with cheap 30wt "flushing oil",run it for 10 miles or so,drain the flushing oil and refill with quality single weight Castrol or Pennzoil for another 1,500 miles.
My experience operating a machine shop catering to historic vehicles has dictated this.

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