Whilst problem shooting my failing boost i changed the temp sender, when i removed the old one i found what looks like french mustard. Expansion tank was the same. I have flushed it through with a bit of soap powder and loads of **** came out. Have refilled the system with a bars leaks shot, the expansion tanks is greasy looking and brown after 10miles but it may be old **** breaking down. I have had no water in the sump or engine- no mayo no water usage. Question is, does this sound like head gasket has gone? Or maybe oil cooler seal.