I'd be grateful to know if ya`ll kin tell me if... k type copper pipe will take open flame heating, enough to heat a circulating oil,to 400 degrees C

It probably will but I would use L type; also pressure rating is much higher. M is bettee still.

Amchiller I think you got your letters backwards or I am wrong about pressures. Type M and type L are thinner wall copper than Type K therefore I think the pressure rating will be higher.
Yes I think you could heat the oil because the melting point of copper is much much higher that the 400 degrees you talked about. Someone correct me if I am wrong.