Correct, the passage through the block is a right angle downward to the sump. What gets clogged is the horizontal part that you can see, as it is the 'coldest' part of the passage. Running the engine with the oil at its high temperature mark will (should) make the downward passage warm enough to keep the oil/sludge liquid and flowing.

The design of the oil separator box, with its baffles, is supposed to condense the oil vapors back to a liquid form so that it can flow back to the sump. A 'cold' engine will not generate enough heat to make the oil flow quickly and sludge builds up - especially true with non-synthetic. The same thing happens with the PTC nipple at the intake.

Even with synthetic, some sludge will build up over time, it cannot be helped. No one can average 75mph over 180,000 miles, nor will people down shift to 2nd or 3rd in city traffic and only get 10mpg.

I cringed when I brought home my current 1995 850T and looked under the oil filler cap. Everything was caramelized! I don't think it was ever subjected to synthetic oil and oil changes were not part of 'regular' maintenance. It is much better now. And still cleaning.
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Keeping it running is better than buying new