Sounds suspiciously like an injector is not closing properly.
When you shut down, the remaining fuel line pressure ( 50-60 pounds ) could force fuel into an open intake valve into the combustion chamber and leak by the piston into the crankcase.


I had this happen on my 1990 GMC Suburban once, caught it before major damage and found the problem by luck - had the air cleaner off and was looking at the injectors as my wifey turned off the ignition. One of the 2 throttle body injectors just kept on spraying ......
( I eventually did have to replace the engine with a new long block, but not before 3 years later )