I've seen both sides of the flush debate: one argument used to be that residue of the flushing fluid may not be compatible with the new oil.
The old oil is black and cacky prciseley because its been doing its job: cleaning and entrapping combustion deposits etc. So long as the oil has been regularly changed then flushing shouldn't be required.
Purists will say flush and drain with the same oil that you will be refilling with. Purists are usually rich, with Aston Martins etc however.
Pointless Engineering Fact: with Babbitt ( white) metal faced bearings: big ends, mains etc, the main function of the oil is cooling: the functional ratio is 80% cooling by carrying the heat away and 20% lubrication via the oil film between journal and shell. White metal is typically an alloy of lead, tin and antimony with other metallurgical odds and sods bunged in. Formulated by a bloke called Babbitt.
It will be all right in the end. If it isn't all right yet, then it is not yet the end..