Okay, I have a problem. It's leaking in the inside of the plug and for some reason the hex is rounded, can't get a grip with a 22mm socket, it will slip. What to do now?

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).