Investigation further up indicates your suggestion about the breather are correct considering the same kind of leak and contamination where the breather hose connects to the air inlet pipe..

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Any further suggestion on how to solve it?
Clean out the breather unit and intercooler and hope it won't happen again for some time.
I have read some suggestions about adding an oil collecting canister or a DIY solution with some hose and a plastic bottle..
Can the leak in the edge of the intercooler be sealed?
Epoxy around the joints?
2006 C5 1.6 HDi ( vin: VF7**************[VIN obfuscated, can be read by forum staff] )
2008 Grand Espace 2.0Dci Auto