I opened up the sump of the cold saw today and was quite amazed

Drained everything and spooned the goop out as well, and made a new
mix with new oil, but I am concerned about this crap in the sump.

Should I replace the oil periodically? (which is dirt cheap)

With 20:1 dilution, and $20 per gallon, it amounts to $0.95 per gallon of
solution.

Although it's cheap, you can make your own (which as a fellow scrounge, may
appeal to you). The recipe was in the Home Shop Machinist some time back in
the early 80s. I don't have the recipe / proportions at hand, but having
made a batch at the time, the secret was to make a concentrated detergent
solution and then slowly add the oil to it while stirring. It didn't work
if you tried to put the detergent solution into the oil.

RWL