notice about oil cooler :

Always check that the nut that secures the oil cooler is tight. ( 27 mm
hex nut ).

They can get loose, somebody changes the oil . . .
the tightened new oil filter sorta holds the oil cooler in place, but
also sorta doesn't.

For the Porsche 944 the Haynes manual says if a-frz gets in the oil "
replace the rod bearings. "  ( ! )

. . . . like that was a simple as replacing the spark plugs.

 Scott

good call on the oil cooler being the possible culprit.