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The passage venting the crankcase blocked, so it cannot breathe. Blow-by gas has to leave the crankcase, and when the route is closed pressure builds until it finds the weakest seal.

So the symptom arrives as an oil leak — past the crankshaft seals, the dipstick tube or the cam cover — and replacing that seal without relieving the pressure means the new one leaks too. A leak that returns quickly after a competent repair is this, which is the most useful thing to know about it. Sludge is the usual blockage, from short journeys and extended oil intervals, so finding it says something about the engine's history. Check crankcase pressure at the oil filler — a quick test that settles it.