Overfilled the oil? 4qts max on stock pan!
The only other thing that I can think of is a problem that I saw way back. An engine compression tested fine and made wonderful power, but filled the catch can in an evening. Car was eventually parted and found that there was a head gasket leak between the cylinder and crankcase. It would hold back the 150 peak psi from a compression test, but the 1500psi from combustion would leak through. Because of the path, there were no blown head gasket symptoms.
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