[pbanders]

Sounds like a crankcase ventilation problem. You got a backfire, which because of either a missing or defective flashback arrestor, or a missing or defective PCV valve, allowed the flame front to propagate to the heads or to the crankcase, where it ignited the hydrocarbon vapors from the oil. It's more likely after lots of cranking, because plenty of the gas is getting past the rings and/or valve guides, and is getting into the oil.