Oil residue in expansion tank

[OmarGrand]

Hey watsup guys, I had an issue where my expansion tank cracked and leaked fluid I also seen oil residue. BMW checked the head gasket also did a compression test everything checked out. Checked the expansion tank this morning seen some oil on the bottom of the cap (nothing milky). I have a friend with a 335 stated it mayb coming from one the lines to the oils cooler any insight would be helpful