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Here's an idea, born of an experience I had with a 1960 Ford 390 V8 many decades ago.
As the engine revs are dropping down to idle, manifold vacuum is at its highest. Somehow there is oil being sucked into the intake system.
While you are driving at a "normal" speed, the vacuum is not as high and/or the oil burns off at a lower rate, so any exhaust change would be hard to detect. But, at idle, the oil builds up, so when you accelerate there is a good bit of oil, and it makes that gawdawful cloud of smoke out the rear.
This theory would also mean that you don't see that cloud when first starting a cold engine in the morning. Is there a cloud then?
Why the knock? Well, I would guess that if your usual gasoline had enough oil vapors blended with it, the octane rating would drop substantially.
Check the entire PCV system for excess oil. You might also run a compression test, including the adding of oil to find weak rings, to find out if you have excessive blow-by. Have a look at the plugs, too. The problem should leave them black and sooty, I would think. Another source of oil would be leaky valve guides.
Let us know what you find out.
Good Luck,
Bob
:>)
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